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{{Short description|Economic model of price determination in a market}}
] model describes how prices vary as a result of a balance between product availability at each price (supply) and the desires of those with purchasing power at each price (demand). The graph depicts an increase in demand from D<sub>1</sub> to D<sub>2</sub> along with the consequent increase in price and quantity required to reach a new market-clearing equilibrium point on the supply curve (S).]]
{{Other uses}}
{{more citations needed|date=January 2021}}
] of price and quantity sold]]
{{Economics sidebar}}
{{Capitalism sidebar}}
] as connected supply and demand curves]]
In ], '''supply and demand''' is an ] of ] in a ]. It postulates that, ], the ] for a particular ] or other traded item in a ], will vary until it settles at the ], where the quantity demanded equals the quantity supplied such that an ] is achieved for price and quantity transacted. The concept of supply and demand forms the theoretical basis of modern economics.


In situations where a firm has ], its decision on how much output to bring to market influences the market price, in violation of perfect competition. There, a more complicated model should be used; for example, an ] or ] model. Likewise, where a buyer has market power, models such as ] will be more accurate.
In ] ], the partial ] '''supply and demand''' ] originally developed by ] attempts to describe, explain, and ] changes in the ] and quantity of ] sold in ] ]s. The model represents a first approximation for describing a market that is not perfectly competitive. It formalizes the theories used by some economists before Marshall and is one of the most fundamental models of some modern economic schools, widely used as a basic building block in a wide range of more detailed ] models and theories. The theory of ] and ] is important for some economic schools understanding of a ] in that it is an explanation of the mechanism by which many ] decisions are made. However, unlike ] models, supply schedules in this ] model are fixed by unexplained forces.


In ], as well, the ] has been used to depict how the quantity of ] and the ] may be determined in equilibrium.


==Graphical representations==
== Assumptions and definitions ==
===Supply schedule===
The theory of supply and demand usually assumes that markets are ]. This implies that there are many buyers and sellers in the market and none of them have the capacity to influence the price of the good.
A supply schedule, depicted graphically as a supply curve, is a table that shows the relationship between the price of a good and the quantity supplied by producers. Under the assumption of ], supply is determined by ]: Firms will produce additional output as long as the cost of extra production is less than the market price.
In many real life transactions, the assumption fails because some individual buyers or sellers or groups of buyers or sellers do have enough ability to influence prices. Quite often a sophisticated analysis is required to understand the demand-supply equation of a good.However, the theory works well in simple situations.


A rise in the cost of raw materials would decrease supply, shifting the supply curve to the left because at each possible price a smaller quantity would be supplied. This shift may also be thought of as an upwards shift in the supply curve, because the price must rise for producers to supply a given quantity. A fall in production costs would increase supply, shifting the supply curve to the right and down.
Mainstream economics does not assume ] that markets are preferable to other forms of social organization. In fact, much analysis is devoted to cases where so-called ]s lead to resource allocation that is suboptimal by some standard. In such cases, economists may attempt to find policies that will avoid waste; directly by government control, indirectly by regulation that induces market participants to act in a manner consistent with optimal welfare, or by creating 'missing' markets to enable efficient trading where none had previously existed. This is studied in the field of ].


Mathematically, a supply curve is represented by a supply function, giving the quantity supplied as a function of its price and as many other variables as desired to better explain quantity supplied. The two most common specifications are:
===Demand===
'''Demand''' is that quantity of a good that consumers are not only willing to buy but also have the capacity to buy at the given price.
For example, a consumer may be willing to purchase 2 lbs of ] if the price is $0.75 per lb. However, the same consumer may be willing to purchase only 1 lb. if the price is $1.00 per lb. A demand schedule can be constructed that shows the quantity demanded at each given price. It can be represented on a graph as a line or curve by plotting the quantity demanded at each price. It can also be described mathematically by a demand equation. The main determinants of the quantity one is willing to purchase will typically be the price of the good, one's level of income, personal tastes, the price of ]s, and the price of ]s.


1) linear supply function, e.g., the slanted line
===Supply===
:<math> Q(P) = 3P - 6 </math>, and
'''Supply''' is the quantity that producers are willing to sell at a given price. For example, the potato grower may be willing to sell 1 million lbs of potatoes if the price is $0.75 per lb and substantially more if the market price is $0.90 per lb. The main determinants of supply will be the market price of the good and the cost of producing it. In fact, supply curves are constructed from the firm's long-run cost schedule.


2) the constant-]<ref>The ], or often just ''elasticity'', is an important parameter in ], used to express the local response of an enzyme or other chemical reaction to changes in its environment.</ref> supply function (also called ] or log-log or loglinear supply function), e.g., the smooth curve
==Simple supply and demand curves==
:<math> Q(P) = 5P^{0.5} </math>
which can be rewritten as
:<math> \log Q(P) = \log 5 + 0.5 \log P </math>


The concept of a supply curve assumes that firms are perfect competitors, having no influence over the market price. This is because each point on the supply curve answers the question, "If this firm is faced with this potential price, how much output will it sell?" If a firm has market power—in violation of the perfect competitor model—its decision on how much output to bring to market influences the market price. Thus the firm is not "faced with" any given price, and a more complicated model, e.g., a ] or ] or ] model, should be used.
Mainstream economic theory centers on creating a series of supply and demand relationships, describing them as ]s, and then adjusting for factors which produce "stickiness" between supply and demand. Analysis is then done to see what "trade offs" are made in the "market" which is the negotiation between sellers and buyers. Analysis is done as to what point the ability of sellers to sell becomes less useful than other opportunities. This is related to "marginal" costs - or the price to produce the last unit that can be sold profitably, versus the chance of using the same effort to engage in some other activity.


Economists distinguish between the supply curve of an individual firm and the market supply curve. The market supply curve shows the total quantity supplied by all firms, so it is the sum of the quantities supplied by all suppliers at each potential price (that is, the individual firms' supply curves are added horizontally).
]


Economists distinguish between short-run and long-run supply curve. ''Short run'' refers to a time period during which one or more inputs are fixed (typically ]), and the number of firms in the industry is also fixed (if it is a market supply curve). ''Long run'' refers to a time period during which new firms enter or existing firms exit andall inputs can be adjusted fully to any price change. Long-run supply curves are flatter than short-run counterparts (with quantity more sensitive to price, more elastic supply).
The slope of the demand curve (downward-to-the-right) indicates that a greater quantity will be demanded when the price is lower. On the other hand, the slope of the supply curve (upward-to-the-right) tells us that as the price goes up, producers are willing to produce more goods. The point where these curves intersect is the ''']'''. At a price of <tt>P</tt> producers will be willing to supply <tt>Q</tt> units per period of time and buyers will demand the same quantity. <tt>P</tt> in this example, is the equilibriating price that equates supply with demand.


Common determinants of supply are:
In the figures, straight lines are drawn instead of the more general curves. This is typical in analysis looking at the simplified relationships between supply and demand because the shape of the curve does not change the general relationships and lessons of the supply and demand theory. The shape of the curves far away from the equilibrium point are less likely to be important because they do not affect the market clearing price and will not affect it unless large shifts in the supply or demand occur. So straight lines for supply and demand with the proper slope will convey most of the information the model can offer. In any case, the exact shape of the curve is not easy to determine for a given market. The general shape of the curve, especially its slope near the equilibrium point, does however have an impact on how a market will adjust to changes in demand or supply. See the below section on ].


# Prices of inputs, including wages
It should be noted that on supply and demand curves both are drawn as a ] of price. Neither is represented as a function of the other. Rather the two functions interact in a manner that is representative of market outcomes. The curves also imply a somewhat neutral means of measuring price. In practice any currency or commodity used to measure price is also the subject of supply and demand.
# The technology used, ]
# Firms' expectations about future prices
# Number of suppliers (for a market supply curve)


===Demand schedule===
===Effects of being away from the equilibrium point===
A demand schedule, depicted graphically as a ], represents the amount of a certain ] that buyers are willing and able to purchase at various prices, assuming all other determinants of demand are held constant, such as income, tastes and preferences, and the prices of ] and ]s. Generally, consumers will buy an additional unit as long as the marginal value of the extra unit is more than the market price they pay. According to the ], the demand curve is always downward-sloping, meaning that as the price decreases, consumers will buy more of the good.
]
Now consider how prices and quantities not at the equilibrium point tend to move towards the equilibrium. Assume that some organization (say government or industry cartel) has the ability to set prices. If the price is set too high, such as at <tt>P1</tt> in the diagram to the right, then the quantity produced will be <tt>Qs</tt>. The quantity demanded will be <tt>Qd</tt>. Since the quantity demanded is less than the quantity supplied there will be an oversupply (also called surplus or excess supply). On the other hand, if the price is set too low, then too little will be produced to meet demand at that price. This will cause an undersupply problem (also called a shortage).


Mathematically, a demand curve is represented by a demand function, giving the quantity demanded as a function of its price and as many other variables as desired to better explain quantity demanded. The two most common specifications are linear demand, e.g., the slanted line
Now assume that individual firms have the ability to alter the quantities supplied and the price they are willing to accept, and consumers have the ability to alter the quantities that they demand and the amount they are willing to pay. Businesses and consumers will respond by adjusting their price (and quantity) levels and this will eventually restore the quantity and the price to the equilibrium.
:<math> Q(P) = 32 - 2P </math>
]
and the constant-] demand function (also called ] or log-log or loglinear demand function), e.g., the smooth curve
In the case of too high a price and oversupply, (seen in the diagram at the left) the profit maximizing businesses will soon have too much excess inventory, so they will lower prices (from P1 to P) to reduce this. Quantity supplied will be reduced from Qs to Q and the oversupply will be eliminated. In the case of too low a price and undersupply, consumers will likely compete to obtain the good at the low price, but since more consumers would like to buy the good at the price that is too low, the profit maximizing firm would raise the price to the highest they can, which is the equilibrium point. In each case, the actions of independent market participants cause the quantity and price to move towards the equilibrium point.
:<math> Q(P) = 3P^{-2} </math>
<BR><BR><BR>
which can be rewritten as
:<math> \log Q(P) = \log 3 - 2 \log P </math>


As a matter of historical convention, a demand curve is drawn with price on the vertical ''y''-axis and demand on the horizontal ''x''-axis. In keeping with modern convention, a demand curve would instead be drawn with price on the ''x''-axis and demand on the ''y''-axis, because price is the independent variable and demand is the variable that is dependent upon price.
==Demand curve shifts==
]
When more people want something, the quantity demanded at all prices will tend to increase. This can be referred to as an ''increase in demand''. The increase in demand could also come from changing tastes, where the same consumers desire more of the same good than they previously did. Increased demand can be represented on the graph as the curve being shifted right, because at each price point, a greater quantity is demanded. An example of this would be more people suddenly wanting more coffee. This will cause the demand curve to shift from the initial curve <tt>D0</tt> to the new curve <tt>D1</tt>. This raises the equilibrium price from <tt>P0</tt> to the higher <tt>P1</tt>. This raises the equilibrium quantity from <tt>Q0</tt> to the higher <tt>Q1</tt>. In this situation, we say that there has been an ''increase'' in demand which has caused an ''extension'' in supply.


Just as the supply curve parallels the ] curve, the demand curve parallels ], measured in dollars.<ref>{{cite web|title=Marginal Utility and Demand|url=http://www.amosweb.com/cgi-bin/awb_nav.pl?s=wpd&c=dsp&k=marginal+utility+and+demand|access-date=2007-02-09}}</ref> Consumers will be willing to buy a given quantity of a good, at a given price, if the marginal utility of additional consumption is equal to the ] determined by the price, that is, the marginal utility of alternative consumption choices. The demand schedule is defined as the ''willingness'' and ''ability'' of a consumer to purchase a given product at a certain time.
Conversely, if the demand decreases, the opposite happens. If the demand starts at <tt>D1</tt>, and then ''decreases'' to <tt>D0</tt>, the price will decrease and the quantity supplied will decrease - a ''contraction'' in supply. Notice that this is purely an effect of demand changing. The quantity supplied at each price is the same as before the demand shift (at both Q0 and Q1). The reason that the equilibrium quantity and price are different is the demand is different.


The demand curve is generally downward-sloping, but for some goods it is upward-sloping. Two such types of goods have been given definitions and names that are in common use: ]s, goods which because of fashion or ] are more attractive at higher prices, and ]s, which, by virtue of being ] that absorb a large part of a consumer's income (e.g., ] such as the classic example of potatoes in Ireland), may see an increase in quantity demanded when the price rises. The reason the law of demand is violated for Giffen goods is that the rise in the price of the good has a strong ], sharply reducing the purchasing power of the consumer so that he switches away from luxury goods to the Giffen good, e.g., when the price of potatoes rises, the Irish peasant can no longer afford meat and eats more potatoes to cover for the lost calories.
==Supply curve shifts==
]
When the suppliers' costs change the supply curve will shift. For
example, assume that someone invents a better way of growing ] so that the amount of wheat that can be grown for a given cost will increase.
Producers will be willing to supply more wheat at every price and this shifts the supply curve <tt>S0</tt> to the right, to <tt>S1</tt> - an ''increase in supply''. This causes the equilibrium price to
decrease from <tt>P0</tt> to <tt>P1</tt>. The equilibrium quantity increases
from <tt>Q0</tt> to <tt>Q1</tt> as the quantity demanded increases at the new lower prices. Notice that in the case of a supply curve shift, the price and the quantity move in opposite directions.
Conversely, if the quantity supplied ''decreases'', the opposite happens. If the supply curve starts at <tt>S1</tt>, and then shifts to <tt>S0</tt>, the equilibrium price will increase and the quantity will decrease. Notice that this is purely an effect of supply changing. The quantity demanded at each price is the same as before the supply shift (at both <tt>Q0</tt> and <tt>Q1</tt>). The reason that the equilibrium quantity and price are different is the ''supply'' is different.


As with the supply curve, the concept of a demand curve requires that the purchaser be a perfect competitor—that is, that the purchaser have no influence over the market price. This is true because each point on the demand curve answers the question, "If buyers are ''faced with'' this potential price, how much of the product will they purchase?" But, if a buyer has market power (that is, the amount he buys influences the price), he is not "faced with" any given price, and we must use a more complicated model, of ].
Another way to view this is that the supply curve moves up and down as opposed to left and right (respectively). If the ability to produce increases as compared to a steady price, the supply shifts up (as opposed to left). If the ability to produce decreases, the supply curve shifts down (as opposed to right). This is a more intuitive approach for beginners, but it is unfortunately not commonly taught.


As with supply curves, economists distinguish between the demand curve for an individual and the demand curve for a market. The market demand curve is obtained by adding the quantities from the individual demand curves at each price.
'''See also:''' ]


Common determinants of demand are:
==Market 'clearance'==
# Income
# Tastes and preferences
# Prices of related goods and services
# Consumers' expectations about future prices and incomes
# Number of potential consumers
# Advertising


===History of the curves===
The market 'clears' at the point where all the supply and demand at a given price balance. That is, the amount of a commodity available at a given price equals the amount that buyers are willing to purchase at that price. It is assumed that there is a process that will result in the market reaching this point, but exactly what the process is in a real situation is an ongoing subject of research. Markets which do not clear will react in some way, either by a change in price, or in the amount produced, or in the amount demanded. Graphically the situation can be represented by two curves: one showing the price-quantity combinations buyers will pay for, or the ]; and one showing the combinations sellers will sell for, or the ]. The market clears where the two are in equilibrium, that is where the curves intersect. In a ] model, all markets in all goods clear simultaneously and the 'price' can be described entirely in terms of tradeoffs with other goods. For a century most economists believed in ], which states that markets, as a whole, would always clear and thus be in balance.
{{multiple image
| width1 = 200
| width2 = 360
| width3 = 200
| align = center
| footer = Figure 2. Early supply and demand curves
| image1 = Cournotdemand.gif
| caption1 = Cournot's ''Recherches'' (1838)
| image2 = Jenkincurves.gif
| caption2 = Jenkin's ''Graphical Representation'' (1870)
| image3 = Marshalldemand.gif
| caption3 = Marshall's ''Principles'' (1890)
}}
Since supply and demand can be considered as ] of price they have a natural graphical representation. Demand curves were first drawn by ] in his {{lang|fr|Recherches sur les Principes Mathématiques de la Théorie des Richesses}} (1838){{snd}}see ]. Supply curves were added by ] in ''The Graphical Representation of the Laws of Supply and Demand...'' of 1870. Both sorts of curve were popularised by ] who, in his '']'' (1890), chose to represent price{{snd}}normally the independent variable{{snd}}by the vertical axis; a practice which remains common.


If supply or demand is a function of other variables besides price, it may be represented by a family of curves (with a change in the other variables constituting a shift between curves) or by a surface in a higher dimensional space.
==Elasticity==
''Main article'': ]


==Microeconomics==
An important concept in understanding supply and demand theory is '''elasticity'''. In this context, it refers to how supply and demand change in response to various stimuli. One way of defining elasticity is the percentage change in one variable divided by the percentage change in another variable (known as ''arch elasticity'' because it calculates the elasticity over a range of values - This can be contrasted with ''point elasticity'' that uses differential calculus to determine the elasticity at a specific point). Thus it is a measure of ''relative'' changes.


===Equilibrium===
Often, it is useful to know how the quantity supplied or demanded will change when the price changes. This is known as the ''']''' and the ''']'''. If a ] decides to increase the price of their product, how will this affect their sales revenue? Will the increased unit price offset the likely decrease in sales volume? If a government imposes a ] on a good, thereby increasing the effecive price, how will this affect the quantity demanded?
Generally speaking, an equilibrium is defined to be the price-quantity pair where the quantity demanded is equal to the quantity supplied. It is represented by the intersection of the demand and supply curves. The analysis of various equilibria is a fundamental aspect of ].


====Market equilibrium====
If you do not wish to calculate elasticity, a simpler technique is to look at the slope of the curve. Unfortunately, this has units of measurement of quantity over monetary unit (For example, ]s per ], or ]s per million ]), which is not a convenient measure to use for most purposes. So, for example if you wanted to compare the effect of a price change of ] in ] versus the ], there is a complicated conversion between ]s per ] and liters per euro. This is one of the reasons why economists often use relative changes in percentages, or elasticity. Another reason is that elasticity is more than just the slope of the function: It is the slope of a function in a coordinate space, that is, a line with a constant slope will have different elasticity at various points.
A situation in a market when the price is such that the quantity demanded by consumers is correctly balanced by the quantity that firms wish to supply. In this situation, the market clears.<ref>{{cite book |last1=Mankiw |first1=N.G. |last2=Taylor |first2=M.P. |date=2011 |title=Economics (2nd ed., revised ed.) |location=Andover |publisher=Cengage Learning}}</ref>


====Changes in market equilibrium====
Lets do an example calculation. We have said that one way of calculating elasticity is the percentage change in quantity over the percentage change in price. So, if the price moves from $1.00 to $1.05, and the quantity supplied goes from 100 pens to 102 pens, the slope is 2/0.05 or 40 pens per dollar. Since the elasticity depends on the percentages, the quantity of pens increased by 2%, and the price increased by 5%, so the elasticity is 2/5 or 0.4.
Practical uses of supply and demand analysis often center on the different variables that change equilibrium price and quantity, represented as shifts in the respective curves. ] of such a shift traces the effects from the initial equilibrium to the new equilibrium.


====Demand curve shifts====
Since the changes are in percentages, changing the unit of measurement or the currency will not affect the elasticity. If the quantity demanded or supplied changes a lot when the price changes a little, it is said to be elastic. If the quantity changes little when the prices changes a lot, it is said to be inelastic. An example of perfectly inelastic supply, or zero elasticity, is represented as a ]. (See that section below)
{{Main|Demand curve}}
] increases both price and quantity.]]
When consumers increase the quantity demanded ''at a given price'', it is referred to as an ''increase in demand''. Increased demand can be represented on the graph as the curve being shifted to the right. At each price point, a greater quantity is demanded, as from the initial curve {{math|''D''<sub>1</sub>}} to the new curve {{math|''D''<sub>2</sub>}}. In the diagram, this raises the equilibrium price from {{math|''P''<sub>1</sub>}} to the higher {{math|''P''<sub>2</sub>}}. This raises the equilibrium quantity from {{math|''Q''<sub>1</sub>}} to the higher {{math|''Q''<sub>2</sub>}}. (A movement along the curve is described as a "change in the quantity demanded" to distinguish it from a "change in demand", that is, a shift of the curve.) The ''increase'' in demand has caused an increase in (equilibrium) quantity. The increase in demand could come from changing tastes and fashions, incomes, price changes in complementary and substitute goods, market expectations, and number of buyers. This would cause the entire demand curve to shift changing the equilibrium price and quantity. Note in the diagram that the shift of the demand curve, by causing a new equilibrium price to emerge, resulted in ''movement along'' the supply curve from the point {{math|(''Q''<sub>1</sub>, ''P''<sub>1</sub>)}} to the point {{math|(''Q''<sub>2</sub>, ''P''<sub>2</sub>)}}.


If the ''demand decreases'', then the opposite happens: a shift of the curve to the left. If the demand starts at {{math|''D''<sub>2</sub>}}, and ''decreases'' to {{math|''D''<sub>1</sub>}}, the equilibrium price will decrease, and the equilibrium quantity will also decrease. The quantity supplied at each price is the same as before the demand shift, reflecting the fact that the supply curve has not shifted; but the equilibrium quantity and price are different as a result of the change (shift) in demand.
Elasticity in relation to variables other than price can also be considered. One of the most common to consider is ]. How would the demand for a good change if income increased or decreased? This is known as the ''']'''. For example how much would the demand for a luxury ] increase if average income increased by 10%? If it is positive, this increase in demand would be represented on a graph by a positive shift in the demand curve, because at all price levels, a greater quantity of luxury cars would be demanded.


====Supply curve shifts====
Another elasticity that is sometimes considered is the ''']''' which measures the responsiveness of the quantity demanded of a good to a change in the price of another good. This is often considered when looking at the relative changes in demand when studying ''']''' and ''']s'''. Complement goods are goods that are typically utilized together, where if one is consumed, usually the other is also. Substitute goods are those where one can be substituted for the other and if the price of one good rises, one may purchase less of it and instead purchase its substitute.
{{Main|Supply (economics)}}
] decreases price and increases quantity.]]
When technological progress occurs, the supply curve shifts. For example, assume that someone invents a better way of growing wheat so that the cost of growing a given quantity of wheat decreases. <!-- DO NOT wikilink "wheat" as this is contrary to Misplaced Pages:Manual of Style/Linking#An example article --> Otherwise stated, producers will be willing to supply more wheat at every price and this shifts the supply curve {{math|''S''<sub>1</sub>}} outward, to {{math|''S''<sub>2</sub>}}—an ''increase in supply''. This increase in supply causes the equilibrium price to decrease from {{math|''P''<sub>1</sub>}} to {{math|''P''<sub>2</sub>}}. The equilibrium quantity increases from {{math|''Q''<sub>1</sub>}} to {{math|''Q''<sub>2</sub>}} as consumers move along the demand curve to the new lower price. As a result of a supply curve shift, the price and the quantity move in opposite directions. If the quantity supplied ''decreases'', the opposite happens. If the supply curve starts at {{math|''S''<sub>2</sub>}}, and shifts leftward to {{math|''S''<sub>1</sub>}}, the equilibrium price will increase and the equilibrium quantity will decrease as consumers move along the demand curve to the new higher price and associated lower quantity demanded. The quantity demanded at each price is the same as before the supply shift, reflecting the fact that the demand curve has not shifted. But due to the change (shift) in supply, the equilibrium quantity and price have changed.


Cross elasticity of demand is measured as the percentage change in demand for the first good that occurs in response to a percentage change in price of the second good. For an example with a complement good, if, in response to a 10% increase in the price of fuel, the quantity of new cars demanded decreased by 20%, the cross elasticity of demand would be -20%/10% or, -2. The movement of the supply curve in response to a change in a non-price determinant of supply is caused by a change in the y-intercept, the constant term of the supply equation. The supply curve shifts up and down the y axis as non-price determinants of demand change.


===Partial equilibrium===
==Vertical supply curve==
{{main|Partial equilibrium}}
Partial equilibrium, as the name suggests, takes into consideration only a part of the market to attain equilibrium.


Jain proposes (attributed to ]): "A partial equilibrium is one which is based on only a restricted range of data, a standard example is price of a single product, the prices of all other products being held fixed during the analysis."<ref>{{cite book|last=Jain|first=T.R.|title=Microeconomics and Basic Mathematics|year=2006–2007|publisher=VK Publications|location=New Delhi|isbn=978-81-87140-89-4|page=28|url=https://books.google.com/books?id=fUUoFwco2Z8C}}{{Dead link|date=August 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
It is sometimes the case that the supply curve is vertical: that is the quantity supplied is fixed, no matter what the market price. For example, the amount of land in the world can be considered fixed. In this case, no matter how much someone would be willing to pay for a piece of land, the extra cannot be created. Also, even if no one wanted all the land, it still would exist. These conditions create a vertical supply curve, giving it zero elasticity (ie. - no matter how large the change in price, the quantity supplied will not change).


The supply-and-demand model is a '''partial equilibrium''' model of ], where the clearance on the ] of some specific ] is obtained independently from prices and quantities in other markets. In other words, the prices of all ] and ], as well as ] levels of ]s are constant. This makes analysis much simpler than in a ] model which includes an entire economy.
In the short run near vertical supply curves are even more common. For example, if the ] is next week, increasing the number of seats in the stadium is almost impossible. The supply of tickets for the game can be considered vertical in this case. If the organizers of this event underestimated demand, then it may very well be the case that the price that they set is below the equilibrium price. In this case there will likely be people who paid the lower price who only value the ticket at that price, and people who could not get tickets, even though they would be willing to pay more. If some of the people who value the tickets less sell them to people who are willing to pay more (i.e. scalp the tickets), then the effective price will rise to the equilibrium price.


Here the dynamic process is that prices adjust until supply equals demand. It is a powerfully simple technique that allows one to study ], ] and ]. The stringency of the simplifying assumptions inherent in this approach makes the model considerably more tractable, but may produce results which, while seemingly precise, do not effectively model real world economic
The graph below illustrates a vertical supply curve. When the
phenomena.
<tt>demand 1</tt> is in effect, the price will be <tt>p1</tt>. When
<tt>demand 2</tt> is occurring, the price will be <tt>p2</tt>. Notice
that at both values the quantity is <tt>Q</tt>. Since the supply is fixed, any shifts in demand will only affect price.


Partial equilibrium analysis examines the effects of policy action in creating equilibrium only in that particular sector or market which is directly affected, ignoring its effect in any other market or industry assuming that they being small will have little impact if any.
]


Hence this analysis is considered to be useful in constricted markets.
==Other ]==


] first formalized the idea of a one-period economic equilibrium of the general economic system, but it was French economist ] and English political economist ] who developed tractable models to analyze an economic system.
In a situation in which there are many buyers but a single ''']''' supplier that can adjust the supply or price of a good at will, the monopolist will adjust the price so that his profit is maximised given the amount that is demanded at that price. This price will be higher than in a competitive market. A similar analysis using supply and demand can be applied when a good has a single buyer, a ''']''', but many sellers.


==Other markets==
Where there are both few buyers or few sellers, the theory of supply and demand cannot be applied because both decisions of the buyers and sellers are interdependent - changes in supply can affect demand and vice versa. ''']''' can be used to analyse this kind of situation. See also ''']'''.
The model of supply and demand also applies to various specialty markets.
The model is commonly applied to ]s in the market for ]. The typical roles of supplier and demander are reversed. The suppliers are individuals, who try to sell their labor for the highest price. The demanders of labor are businesses, which try to buy the type of labor they need at the lowest price. The equilibrium price for a certain type of labor is the wage rate.<ref>{{cite web|last=Kibbe|first=Matthew B.|title=The Minimum Wage: Washington's Perennial Myth|publisher=]|url=http://www.cato.org/pubs/pas/pa106.html|access-date=2007-02-09}}</ref> However, economist Steve Fleetwood revisited the empirical reality of supply and demand curves in labor markets and concluded that the evidence is "at best inconclusive and at worst casts doubt on their existence." For instance, he cites Kaufman and Hotchkiss (2006): "For adult men, nearly all studies find the labour supply curve to be negatively sloped or backward bending."<ref>{{cite journal|last1=Fleetwood|first1=Steve|title=Do labour supply and demand curves exist?|journal=Cambridge Journal of Economics|date=August 2014|volume=38|issue=5|pages=1087–113|doi=10.1093/cje/beu003|url=https://www.researchgate.net/publication/275418190}}</ref> Supply and demand can be used to explain ]s,<ref name="h488">{{cite journal | last=Cole | first=A. | title=BMA meeting: Doctors vote to limit number of medical students | journal=BMJ | volume=337 | issue=jul09 1 | date=9 July 2008 | issn=0959-8138 | doi=10.1136/bmj.a748 | pages=a748}}</ref> ]s<ref name="Ariste_2019">{{Cite journal| doi = 10.1002/hpm.2772| issn = 1099-1751| volume = 34| issue = 4| pages = 1144–1154| last1 = Ariste| first1 = Ruolz| last2 = Béjaoui| first2 = Ali| last3 = Dauphin| first3 = Anyck| title = Critical analysis of nurses' labour market effectiveness in Canada: The hidden aspects of the shortage.| journal = The International Journal of Health Planning and Management | date = October 10, 2019| pmid = 30945352| s2cid = 92997538}}</ref> or ]s.<ref name="y456">{{Cite report |url=https://eric.ed.gov/?id=ED606666 |title=A Coming Crisis in Teaching? Teacher Supply, Demand, and Shortages in the U.S. |last1=Sutcher |first1=Leib |last2=Darling-Hammond |first2=Linda |date=September 2016 |publisher=Learning Policy Institute |last3=Carver-Thomas |first3=Desiree |access-date=20 July 2024}}{{page needed|date=July 2024}}</ref>


In both ] and ] economics, the ] is analyzed as a supply-and-demand system with ] being the price. The ] may be a vertical supply curve, if the ] of a country chooses to use ] to fix its value regardless of the interest rate; in this case the money supply is totally inelastic. On the other hand,<ref>Basij J. Moore, ''Horizontalists and Verticalists: The Macroeconomics of Credit Money'', Cambridge University Press, 1988</ref> the money supply curve is a horizontal line if the central bank is targeting a fixed interest rate and ignoring the value of the money supply; in this case the money supply curve is perfectly elastic. The demand for money intersects with the money supply to determine the interest rate.<ref name=RitterSilberUdell>{{cite book|last1 = Ritter|first1 = Lawrence S.|author-link1 = Lawrence S. Ritter|last2 = Silber|first2 = William L.|author-link2 = William L. Silber|last3 = Udell|first3 = Gregory F.|title = Principles of Money, Banking, and Financial Markets|edition = 10th|publisher = Addison-Wesley, Menlo Park C| year = 2000|isbn = 978-0-321-37557-5|pages=431–38, 465–76}}</ref>
The supply curve does not have to be linear. However, if the supply is
from a profit maximizing firm, it can be proven that supply curves are not downward sloping (i.e. if the price increases, the quantity supplied will not decrease). Supply curves from profit maximizing firms can be vertical, horizontal or upward sloping. While it is possible for industry supply curves to be downward sloping, supply curves for individual firms are never downward sloping).


According to some studies,<ref>Velev, Milen V. Entropy and free-energy based interpretation of the laws of supply and demand. SN Bus Econ 1, 1 (2021). https://doi.org/10.1007/s43546-020-00009-6</ref> the laws of supply and demand are applicable not only to the business relationships of people, but to the behaviour of social animals and to all living things that interact on the biological markets<ref>Noë, R., Hammerstein, P. Biological markets: supply and demand determine the effect of partner choice in cooperation, mutualism and mating. Behav Ecol Sociobiol 35, 1–11 (1994). https://doi.org/10.1007/BF00167053</ref> in scarce resource environments.
Standard microeconomic assumptions cannot be used to prove that the demand curve is downward sloping. However, despite years of searching, no generally agreed upon example of a good that has an upward sloping demand curve has been found (also known as a ''']'''). Non-economists sometimes think that certain goods would have such a curve. For example, some people will buy a luxury car because it is expensive. In this case the good demanded is actually ], and not a car, so when the price of the luxury car decreases, it is actually changing the amount of prestige so the demand is not decreasing since it is a different good (see ''']'''). Even with downward sloping demand curves, it is possible that an increase in income may lead to a decrease in demand for a particular good, probably due to the existence of more attractive alternatives which become affordable: a good with this property is known as an ''']'''.


The model of supply and demand accurately describes the characteristic of metabolic systems: specifically, it explains how ] allows metabolic pathways to respond to the demand for a metabolic intermediates while minimizing effects due to variation in the supply.<ref>{{cite journal
==An example: Supply and demand in a 6-person economy==
| last1 = Hofmeyr
| first1 = J.-H.S.
| last2 = Cornish-Bowden
| first2 = A.
| title = Regulating the cellular economy of supply and demand.
| year = 2000
| journal = FEBS Lett.
| volume = 476
| issue = 1
| pages = 47–51
| doi = 10.1111/j.1432-1033.1991.tb21071.x
| pmid = 1879427
| doi-access=
}}</ref>


==Empirical estimation==
Supply and demand can be thought of in terms of individual people interacting at a market. Suppose the following six people participate in this simplified economy:
Demand and supply relations in a market can be statistically estimated from price, quantity, and other ] with sufficient information in the model. This can be done with ''] methods of estimation'' in ]. Such methods allow solving for the model-relevant "structural coefficients," the estimated algebraic counterparts of the theory. The '']'' is a common issue in "structural estimation." Typically, data on ] variables (that is, variables other than price and quantity, both of which are ] variables) are needed to perform such an estimation. An alternative to "structural estimation" is ] estimation, which regresses each of the endogenous variables on the respective exogenous variables.


==Macroeconomic uses==
* Alice is willing to pay $10 for a sack of potatoes.
] worked per week in the United States. ] is supply, money is ].]]
* Bob is willing to pay $20 for a sack of potatoes.
* Cathy is willing to pay $30 for a sack of potatoes.
* Dan is willing to sell a sack of potatoes for $5.
* Emily is willing to sell a sack of potatoes for $15.
* Fred is willing to sell a sack of potatoes for $25.


Demand and supply have also been generalized to explain ] variables in a ], including the ] and the ]. The ] may be the most direct application of supply and demand to macroeconomics, but other macroeconomic models also use supply and demand. Compared to ] uses of demand and supply, different (and more controversial) theoretical considerations apply to such ] counterparts as ] and ]. Demand and supply are also used in macroeconomic theory to relate ] and money demand to ]s, and to relate labor supply and labor demand to wage rates.
There are many possible trades that would be mutually agreeable to both people, but not all of them will happen. For example, Cathy and Fred would be interested in trading with each other for any price between $25 and $30. If the price is above $30, Cathy is not interested, since the price is too high. If the price is below $25, Fred is not interested since the price is too low. However at the market, Cathy will discover that there are other sellers willing to sell at well below $25, so she will not trade with Fred at all. In an efficient market, each seller will get as high a price as possible, and each buyer will get as low a price as possible.


==History==
Imagine that Cathy and Fred are bartering over the price. Fred offers $25 for a sack of potatoes. Before Cathy can agree, Emily offers a sack of potatoes for $24. Fred is not willing to sell at $24, so he drops out. At this point, Dan offers to sell for $12. Emily won't sell for that amount so it looks like the deal might go through. At this point Bob steps in and offers $14. Now we have two people willing to pay $14 for a sack of potatoes (Cathy and Bob), but only one person (Dan) willing to sell for $14. Cathy notices this, and doesn't want to lose a good deal, so she offers Dan $16 for his potatoes. Now Emily also offers to sell for $16, so there are two buyers and two sellers at that price (note that they could have settled on any price between $15 and $20), and the bartering can stop. But what about Fred and Alice? Well, Fred and Alice are not willing to trade with each other since Alice is only willing to pay $10 and Fred will not sell for any amount under $25. Alice can't outbid Cathy or Bob to purchase from Dan so Alice will not be able to get a trade with them. Fred can't underbid Dan or Emily so he will not be able to get a trade with Cathy. In other words, a stable equilibrium has been reached.
The 256th couplet of ], which was composed at least 2000 years ago, says that "if people do not consume a product or service, then there will not be anybody to supply that product or service for the sake of price".<ref>C Chendroyaperumal (2010). </ref>
]
A supply and demand graph could also be drawn from this. The demand would be:


According to Hamid S. Hosseini, the power of supply and demand was understood to some extent by several early Muslim scholars, such as fourteenth-century Syrian scholar ], who wrote: "If desire for goods increases while its availability decreases, its price rises. On the other hand, if availability of the good increases and the desire for it decreases, the price comes down."<ref name=Hosseini>{{cite book |title=A Companion to the History of Economic Thought |chapter=Contributions of Medieval Muslim Scholars to the History of Economics and their Impact: A Refutation of the Schumpeterian Great Gap |last=Hosseini |first=Hamid S. |editor=Biddle, Jeff E. |editor2=Davis, Jon B. |editor3=Samuels, Warren J. |year=2003 |publisher=Blackwell |location=Malden, MA |isbn=978-0-631-22573-7 |doi=10.1002/9780470999059.ch3 |pages=28–45 }} (citing Hamid S. Hosseini, 1995. "Understanding the Market Mechanism Before Adam Smith: Economic Thought in Medieval Islam," ''History of Political Economy'', Vol. 27, No. 3, 539–61).</ref>
* 1 person is willing to pay $30 (Cathy).
{{Rquote|right|If desire for goods increases while its availability decreases, its price rises. On the other hand, if availability of the good increases and the desire for it decreases, the price comes down.|]|<ref name=Hosseini />}}
* 2 people are willing to pay $20 (Cathy and Bob).
]
* 3 people are willing to pay $10 (Cathy, Bob, and Alice).


Shifting focus to the English etymology of the expression, it has been confirmed that the phrase 'supply and demand' was not used by English economics writers until after the end of the 17th century.<ref>{{cite journal |last1=Thweatt |first1=W.O. |title=Origins of the terminology, supply and demand. |journal=Scottish Journal of Political Economy |date=1983 |volume=30 |issue=3 |pages=287–294|doi=10.1111/j.1467-9485.1983.tb01020.x }}</ref>
The supply would be:
In ]'s 1691 work ''Some Considerations on the Consequences of the Lowering of Interest and the Raising of the Value of Money'',<ref>John Locke (1691) </ref> Locke alluded to the idea of supply and demand, however, he failed to accurately label it as such and thus, he fell short in coining the phrase and conveying its true significance.<ref name="Groenewegen P. 2008">Groenewegen P. (2008) ‘Supply and Demand’. In: Palgrave Macmillan (eds) The New Palgrave Dictionary of Economics. Palgrave Macmillan, London</ref> Locke wrote: “The price of any commodity rises or falls by the proportion of the number of buyer and sellers” and “that which regulates the price... is nothing else but their quantity in proportion to Vent.”<ref name="Groenewegen P. 2008"/> Locke's terminology drew criticism from John Law. Law argued that,"The Prices of Goods are not according to the quantity in proportion to the Vent, but in proportion to the Demand."<ref>{{cite book |last1=Law |first1=J. |title=Money and Trade Considered with a Proposal for Supplying the Nation with Money |date=1705 |publisher=Anderson |location=Edinburgh}}</ref> From Law the demand part of the phrase was given its proper title and it began to circulate among "prominent authorities" in the 1730s.<ref name="Groenewegen P. 2008"/> In 1755, ], in his ''A System of Moral Philosophy'', furthered development toward the phrase by stipulating that, "the prices of goods depend on these two jointly, the Demand... and the Difficulty of acquiring."<ref name="Groenewegen P. 2008"/>


It was not until 1767 that the phrase "supply and demand" was first used by Scottish writer ] in his ''Inquiry into the Principles of Political Economy.'' He originated the use of this phrase by effectively combining "supply" and "demand" together in a number of different occasions such as ] and ]. In Steuart's chapter entitled "Of Demand", he argues that "The nature of Demand is to encourage industry; and when it is regularly made, the effect of it is, that the supply for the most part is found to be in proportion to it, and then the demand is simple". It is presumably from this chapter that the idea spread to other authors and economic thinkers. ] used the phrase after Steuart in his 1776 book '']''. In ''The Wealth of Nations'', Smith asserted that the supply price was fixed but that its "merit" (value) would decrease as its "scarcity" increased, this idea by Smith was later named the law of demand. In 1803, Thomas Robert Malthus used the phrase "supply and demand" twenty times in the second edition of the ''Essay on Population''.<ref name="Groenewegen P. 2008"/>
* 1 person is willing to sell for $5 (Dan).
And ] in his 1817 work, '']'', titled one chapter, "On the Influence of Demand and Supply on Price".<ref name=Humphrey>{{Cite journal
* 2 people are willing to sell for $15 (Dan and Emily).
| issue = Mar/Apr
* 3 people are willing to sell for $25 (Dan, Emily, and Fred).
| pages = 3–23
| last = Humphrey
| first = Thomas M.
| title = Marshallian Cross Diagrams and their Uses before Alfred Marshall
| journal = Economic Review
| date = 1992
| url = https://www.richmondfed.org/-/media/richmondfedorg/publications/research/economic_review/1992/pdf/er780201.pdf
| access-date = 2019-07-27}}</ref> In ''Principles of Political Economy and Taxation'', Ricardo more rigorously laid down the idea of the assumptions that were used to build his ideas of supply and demand. In 1838, ] developed a mathematical model of supply and demand in his ''Researches into the Mathematical Principles of Wealth'', it included diagrams. It is important to note that the use of the phrase was still rare and only a few examples of more than 20 uses in a single work have been identified by the end of the second decade of the 19th century.<ref name="Groenewegen P. 2008"/>


During the late 19th century the marginalist school of thought emerged. The main innovators of this approach were ], ], and ]. The key idea was that the price was set by the subjective value of a good at the margin. This was a substantial change from Adam Smith's thoughts on determining the supply price.
Supply and demand match when the quantity traded is two sacks and the price is between $15 and $20. Whether Dan sells to Cathy, and Emily to Bob, or the other way round, and what precisely is the price agreed cannot be determined. This is the only limitation of this simple model. When considering the full assumptions of perfect competition the price would be fully determined since there would be enough participants to determine the price. For example, if the "last trade" was between someone willing to sell at $15.50 and someone willing to pay $15.51, then the price could be determined to the penny. As more participants enter, the more likely there will be a close bracketing of the equilibrium price.


In his 1870 essay "On the Graphical Representation of Supply and Demand", ] in the course of "introduc the diagrammatic method into the English economic literature" published the first drawing of supply and demand curves in English,<ref>A.D. Brownlie and M. F. Lloyd Prichard, 1963. "Professor Fleeming Jenkin, 1833–1885 Pioneer in Engineering and Political Economy," ''Oxford Economic Papers'', NS, 15(3), p. 211.</ref> including ] from a shift of supply or demand and application to the labor market.<ref>Fleeming Jenkin, 1870. "''The Graphical Representation of the Laws of Supply and Demand, and their Application to Labour,''" in Alexander Grant, ed., (Scroll to chapter) Edinburgh: Edmonston and Douglas</ref> The model was further developed and popularized by ] in the 1890 textbook '']''.<ref name="Humphrey" />
It is important to note that this example violates the assumption of perfect competition in that there are a limited number of market participants. However this simplification shows how the equilibrium price and quantity can be determined in an easily understood situation. The results are similar when unlimited market participants and the other assumptions of perfect competition are considered.


==Decision-making== ==Criticism==
]'s critique focused on the inconsistency (except in implausible circumstances) of partial equilibrium analysis and the rationale for the upward slope of the supply curve in a market for a produced consumption good.<ref>Avi J. Cohen, "'The Laws of Returns Under Competitive Conditions': Progress in Microeconomics Since Sraffa (1926)?", ''Eastern Economic Journal'', V. 9, N. 3 (Jul.-Sep.): 1983)</ref> The notability of Sraffa's critique is also demonstrated by ]'s comments and engagements with it over many years, for example:
:What a cleaned-up version of Sraffa (1926) establishes is how ''nearly empty'' are ''all'' of Marshall's partial equilibrium boxes. To a logical purist of Wittgenstein and Sraffa class, the ''Marshallian partial'' equilibrium box of ''constant'' cost is even more empty than the box of ''increasing'' cost.<ref>Paul A. Samuelson, "Reply" in ''Critical Essays on Piero Sraffa's Legacy in Economics'' (edited by H. D. Kurz) Cambridge University Press, 2000</ref>


Modern ] criticize the supply and demand model for failing to explain the prevalence of ], in which retail prices are set by firms, primarily based on a mark-up over normal average unit costs, and are not responsive to changes in demand up to capacity.<ref>Lee, F. S. (1999). Post Keynesian price theory. Cambridge University Press.</ref>
Much of economics assumes that individuals seek to maximize their happiness or utility: however, whether they ] attempt to optimize their well-being given available information is a source of much debate. In this view, which underpins much of economic writing, individuals make choices between alternatives based on their estimation of which will yield the best results. Many important economic ideas, such as the "efficient market hypothesis" rest on this view of decision making.

However, this framework, once called "]" - has for decades been the focus of unease even by those who apply it. ] once defended the idea by saying that inaccurate assumptions could produce accurate results. Alfred Marshall was careful to differentiate the tendency to maximize happiness, with maximizing economic well being. The limits of rationality have been the subject of intense study, for example ]'s model for "]", which was awarded a ] in ]. More recently, irrational behavior and imperfect information have increasingly been the subject of formal modelling, often referred to as behavioral economics, for which ] won a Nobel Prize in ]. An example is the growing field of ] which combines previous theory with ].

The new model of information and decision making focuses on asymmetrical information, when some participants have key facts that others do not, and on decision making based, not on the economic pressures, but on the decisions of other economic actors. Asymmetrical information and behavioral dynamics lead to different conclusions: in a world of asymmetrical information, markets are generally not efficient, and inefficiences grow up as means of hedging against information. While not yet universally accepted, it is increasingly influential in policy, for example the writing of ] and financial modelling.

==History of supply and demand==

Attempts to determine how supply and demand interact began with ]'s '']'', first published in 1776. In this book, he mostly assumed that the supply price was fixed, but that the demand would increase or decrease as the price decreased or increased. ] in 1817 published the book '']'', in which the first idea of an economic model was proposed. In this, he more rigorously laid down the idea of the assumptions that were used to build his ideas of supply and demand.

During the late 19th century the marginalist school of thought emerged. This field mainly was started by ], ], and ]. The key idea was that the price was set by the most expensive price, i.e. the price at the margin. This was a substantial change from Adam Smith's thoughts on determining the supply price.

Finally, most of the basics of the modern school theory of supply and demand was finalized by ] and ] when they combined the ideas about supply and the ideas about demand and began looking at the equilbrium point where the two curves crossed. They also began looking at the effect of markets on each other. Since the late 19th century, the theory of supply and demand has mainly been unchanged. Most of the work has been in examining the exceptions to the model (like oligarchy, transaction costs, non-rationality).

==Criticism of Marshall's theory of supply and demand==

Marshall's theory of supply and demand runs counter to the ideas of economists from Adam Smith and David Ricardo through the creation of the marginalist school of thought. Although Marshall's theories are dominant in elite universities today, not everyone has taken the fork in the road that he and the marginalists proposed. One theory counter to Marshall is that price is already known in a commodity before it reaches the market, negating his idea that some abstract market is conveying price information. The only thing the market communicates is whether or not an object is exchangable or not (in which case it would change from an object to a commodity). This would mean that the producer creates the goods without already having customers - blindly producing, hoping that someone will buy them (''buy'' meaning exchange money for the commodities). Modern producers often have market studies prepared well in advance of production decisions, however, misallocation of factors of production can still occur.

] also runs counter to the theory of supply and demand. In Keynesian theory, prices can become "sticky" or resistant to change, especially in the case of price decreases. This leads to a market failure. Modern supporters of Keynes, such as ], have noted this in recent history, such as when the ] housing market dried up in the early 1990's, with neither buyers nor sellers willing to exchange at the price equilibrium.

] work on the irrationality of actors in the markets also undermines Marshall's simplistic view of the forces involved in supply and demand.


==See also== ==See also==
{{Div col|colwidth=22em}}

* ]
* ] * ]
* ] * ]
* ] * ]
* ]
* ] * ]
* ] * ]
* ]
* ] * ]
* ] * ]
* ]
* ]
* ]
* ]
* ]
* ] * ]
* ] * ]
* ]
* ] * ]
* ] * ]
{{Div col end}}

==References==
{{Reflist|30em}}

==Further reading==
* '']'' by Paul A. Samuelson
* ''Price Theory and Applications'' by Steven E. Landsburg {{ISBN|0-538-88206-9}}
* '']'', ], 1776
* book by ] at Project Gutenberg.


== External link and references == ==External links==
{{Commons category|Supply and demand curves}}
{{Wiktionary|supply|demand}}
* (])
* by ]
* '''', a brief statement of ]'s rival account
* by Fiona Maclachlan and by Mark Gillis, ].


* book by ] at Project Gutenberg.
*Price Theory and Applications by Steven E. Landsburg ISBN 0-538-88206-9
*''An Inquiry into the Nature and Causes of the Wealth of Nations'', Adam Smith, 1776
*''By what is the price of a commodity determined?'', a brief statement of Karl Marx's rival account
{{microeconomics-footer}} {{microeconomics-footer}}
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Economic model of price determination in a market For other uses, see Supply and demand (disambiguation).
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Supply chain as connected supply and demand curves

In microeconomics, supply and demand is an economic model of price determination in a market. It postulates that, holding all else equal, the unit price for a particular good or other traded item in a perfectly competitive market, will vary until it settles at the market-clearing price, where the quantity demanded equals the quantity supplied such that an economic equilibrium is achieved for price and quantity transacted. The concept of supply and demand forms the theoretical basis of modern economics.

In situations where a firm has market power, its decision on how much output to bring to market influences the market price, in violation of perfect competition. There, a more complicated model should be used; for example, an oligopoly or differentiated-product model. Likewise, where a buyer has market power, models such as monopsony will be more accurate.

In macroeconomics, as well, the aggregate demand-aggregate supply model has been used to depict how the quantity of total output and the aggregate price level may be determined in equilibrium.

Graphical representations

Supply schedule

A supply schedule, depicted graphically as a supply curve, is a table that shows the relationship between the price of a good and the quantity supplied by producers. Under the assumption of perfect competition, supply is determined by marginal cost: Firms will produce additional output as long as the cost of extra production is less than the market price.

A rise in the cost of raw materials would decrease supply, shifting the supply curve to the left because at each possible price a smaller quantity would be supplied. This shift may also be thought of as an upwards shift in the supply curve, because the price must rise for producers to supply a given quantity. A fall in production costs would increase supply, shifting the supply curve to the right and down.

Mathematically, a supply curve is represented by a supply function, giving the quantity supplied as a function of its price and as many other variables as desired to better explain quantity supplied. The two most common specifications are:

1) linear supply function, e.g., the slanted line

Q ( P ) = 3 P 6 {\displaystyle Q(P)=3P-6} , and

2) the constant-elasticity supply function (also called isoelastic or log-log or loglinear supply function), e.g., the smooth curve

Q ( P ) = 5 P 0.5 {\displaystyle Q(P)=5P^{0.5}}

which can be rewritten as

log Q ( P ) = log 5 + 0.5 log P {\displaystyle \log Q(P)=\log 5+0.5\log P}

The concept of a supply curve assumes that firms are perfect competitors, having no influence over the market price. This is because each point on the supply curve answers the question, "If this firm is faced with this potential price, how much output will it sell?" If a firm has market power—in violation of the perfect competitor model—its decision on how much output to bring to market influences the market price. Thus the firm is not "faced with" any given price, and a more complicated model, e.g., a monopoly or oligopoly or differentiated-product model, should be used.

Economists distinguish between the supply curve of an individual firm and the market supply curve. The market supply curve shows the total quantity supplied by all firms, so it is the sum of the quantities supplied by all suppliers at each potential price (that is, the individual firms' supply curves are added horizontally).

Economists distinguish between short-run and long-run supply curve. Short run refers to a time period during which one or more inputs are fixed (typically physical capital), and the number of firms in the industry is also fixed (if it is a market supply curve). Long run refers to a time period during which new firms enter or existing firms exit andall inputs can be adjusted fully to any price change. Long-run supply curves are flatter than short-run counterparts (with quantity more sensitive to price, more elastic supply).

Common determinants of supply are:

  1. Prices of inputs, including wages
  2. The technology used, productivity
  3. Firms' expectations about future prices
  4. Number of suppliers (for a market supply curve)

Demand schedule

A demand schedule, depicted graphically as a demand curve, represents the amount of a certain good that buyers are willing and able to purchase at various prices, assuming all other determinants of demand are held constant, such as income, tastes and preferences, and the prices of substitute and complementary goods. Generally, consumers will buy an additional unit as long as the marginal value of the extra unit is more than the market price they pay. According to the law of demand, the demand curve is always downward-sloping, meaning that as the price decreases, consumers will buy more of the good.

Mathematically, a demand curve is represented by a demand function, giving the quantity demanded as a function of its price and as many other variables as desired to better explain quantity demanded. The two most common specifications are linear demand, e.g., the slanted line

Q ( P ) = 32 2 P {\displaystyle Q(P)=32-2P}

and the constant-elasticity demand function (also called isoelastic or log-log or loglinear demand function), e.g., the smooth curve

Q ( P ) = 3 P 2 {\displaystyle Q(P)=3P^{-2}}

which can be rewritten as

log Q ( P ) = log 3 2 log P {\displaystyle \log Q(P)=\log 3-2\log P}

As a matter of historical convention, a demand curve is drawn with price on the vertical y-axis and demand on the horizontal x-axis. In keeping with modern convention, a demand curve would instead be drawn with price on the x-axis and demand on the y-axis, because price is the independent variable and demand is the variable that is dependent upon price.

Just as the supply curve parallels the marginal cost curve, the demand curve parallels marginal utility, measured in dollars. Consumers will be willing to buy a given quantity of a good, at a given price, if the marginal utility of additional consumption is equal to the opportunity cost determined by the price, that is, the marginal utility of alternative consumption choices. The demand schedule is defined as the willingness and ability of a consumer to purchase a given product at a certain time.

The demand curve is generally downward-sloping, but for some goods it is upward-sloping. Two such types of goods have been given definitions and names that are in common use: Veblen goods, goods which because of fashion or signalling are more attractive at higher prices, and Giffen goods, which, by virtue of being inferior goods that absorb a large part of a consumer's income (e.g., staples such as the classic example of potatoes in Ireland), may see an increase in quantity demanded when the price rises. The reason the law of demand is violated for Giffen goods is that the rise in the price of the good has a strong income effect, sharply reducing the purchasing power of the consumer so that he switches away from luxury goods to the Giffen good, e.g., when the price of potatoes rises, the Irish peasant can no longer afford meat and eats more potatoes to cover for the lost calories.

As with the supply curve, the concept of a demand curve requires that the purchaser be a perfect competitor—that is, that the purchaser have no influence over the market price. This is true because each point on the demand curve answers the question, "If buyers are faced with this potential price, how much of the product will they purchase?" But, if a buyer has market power (that is, the amount he buys influences the price), he is not "faced with" any given price, and we must use a more complicated model, of monopsony.

As with supply curves, economists distinguish between the demand curve for an individual and the demand curve for a market. The market demand curve is obtained by adding the quantities from the individual demand curves at each price.

Common determinants of demand are:

  1. Income
  2. Tastes and preferences
  3. Prices of related goods and services
  4. Consumers' expectations about future prices and incomes
  5. Number of potential consumers
  6. Advertising

History of the curves

Cournot's Recherches (1838)Jenkin's Graphical Representation (1870)Marshall's Principles (1890)Figure 2. Early supply and demand curves

Since supply and demand can be considered as functions of price they have a natural graphical representation. Demand curves were first drawn by Augustin Cournot in his Recherches sur les Principes Mathématiques de la Théorie des Richesses (1838) – see Cournot competition. Supply curves were added by Fleeming Jenkin in The Graphical Representation of the Laws of Supply and Demand... of 1870. Both sorts of curve were popularised by Alfred Marshall who, in his Principles of Economics (1890), chose to represent price – normally the independent variable – by the vertical axis; a practice which remains common.

If supply or demand is a function of other variables besides price, it may be represented by a family of curves (with a change in the other variables constituting a shift between curves) or by a surface in a higher dimensional space.

Microeconomics

Equilibrium

Generally speaking, an equilibrium is defined to be the price-quantity pair where the quantity demanded is equal to the quantity supplied. It is represented by the intersection of the demand and supply curves. The analysis of various equilibria is a fundamental aspect of microeconomics.

Market equilibrium

A situation in a market when the price is such that the quantity demanded by consumers is correctly balanced by the quantity that firms wish to supply. In this situation, the market clears.

Changes in market equilibrium

Practical uses of supply and demand analysis often center on the different variables that change equilibrium price and quantity, represented as shifts in the respective curves. Comparative statics of such a shift traces the effects from the initial equilibrium to the new equilibrium.

Demand curve shifts

Main article: Demand curve
Right-shift of demand curve increases both price and quantity.

When consumers increase the quantity demanded at a given price, it is referred to as an increase in demand. Increased demand can be represented on the graph as the curve being shifted to the right. At each price point, a greater quantity is demanded, as from the initial curve D1 to the new curve D2. In the diagram, this raises the equilibrium price from P1 to the higher P2. This raises the equilibrium quantity from Q1 to the higher Q2. (A movement along the curve is described as a "change in the quantity demanded" to distinguish it from a "change in demand", that is, a shift of the curve.) The increase in demand has caused an increase in (equilibrium) quantity. The increase in demand could come from changing tastes and fashions, incomes, price changes in complementary and substitute goods, market expectations, and number of buyers. This would cause the entire demand curve to shift changing the equilibrium price and quantity. Note in the diagram that the shift of the demand curve, by causing a new equilibrium price to emerge, resulted in movement along the supply curve from the point (Q1, P1) to the point (Q2, P2).

If the demand decreases, then the opposite happens: a shift of the curve to the left. If the demand starts at D2, and decreases to D1, the equilibrium price will decrease, and the equilibrium quantity will also decrease. The quantity supplied at each price is the same as before the demand shift, reflecting the fact that the supply curve has not shifted; but the equilibrium quantity and price are different as a result of the change (shift) in demand.

Supply curve shifts

Main article: Supply (economics)
Right-shift of supply curve decreases price and increases quantity.

When technological progress occurs, the supply curve shifts. For example, assume that someone invents a better way of growing wheat so that the cost of growing a given quantity of wheat decreases. Otherwise stated, producers will be willing to supply more wheat at every price and this shifts the supply curve S1 outward, to S2—an increase in supply. This increase in supply causes the equilibrium price to decrease from P1 to P2. The equilibrium quantity increases from Q1 to Q2 as consumers move along the demand curve to the new lower price. As a result of a supply curve shift, the price and the quantity move in opposite directions. If the quantity supplied decreases, the opposite happens. If the supply curve starts at S2, and shifts leftward to S1, the equilibrium price will increase and the equilibrium quantity will decrease as consumers move along the demand curve to the new higher price and associated lower quantity demanded. The quantity demanded at each price is the same as before the supply shift, reflecting the fact that the demand curve has not shifted. But due to the change (shift) in supply, the equilibrium quantity and price have changed.

The movement of the supply curve in response to a change in a non-price determinant of supply is caused by a change in the y-intercept, the constant term of the supply equation. The supply curve shifts up and down the y axis as non-price determinants of demand change.

Partial equilibrium

Main article: Partial equilibrium

Partial equilibrium, as the name suggests, takes into consideration only a part of the market to attain equilibrium.

Jain proposes (attributed to George Stigler): "A partial equilibrium is one which is based on only a restricted range of data, a standard example is price of a single product, the prices of all other products being held fixed during the analysis."

The supply-and-demand model is a partial equilibrium model of economic equilibrium, where the clearance on the market of some specific goods is obtained independently from prices and quantities in other markets. In other words, the prices of all substitutes and complements, as well as income levels of consumers are constant. This makes analysis much simpler than in a general equilibrium model which includes an entire economy.

Here the dynamic process is that prices adjust until supply equals demand. It is a powerfully simple technique that allows one to study equilibrium, efficiency and comparative statics. The stringency of the simplifying assumptions inherent in this approach makes the model considerably more tractable, but may produce results which, while seemingly precise, do not effectively model real world economic phenomena.

Partial equilibrium analysis examines the effects of policy action in creating equilibrium only in that particular sector or market which is directly affected, ignoring its effect in any other market or industry assuming that they being small will have little impact if any.

Hence this analysis is considered to be useful in constricted markets.

Léon Walras first formalized the idea of a one-period economic equilibrium of the general economic system, but it was French economist Antoine Augustin Cournot and English political economist Alfred Marshall who developed tractable models to analyze an economic system.

Other markets

The model of supply and demand also applies to various specialty markets. The model is commonly applied to wages in the market for labor. The typical roles of supplier and demander are reversed. The suppliers are individuals, who try to sell their labor for the highest price. The demanders of labor are businesses, which try to buy the type of labor they need at the lowest price. The equilibrium price for a certain type of labor is the wage rate. However, economist Steve Fleetwood revisited the empirical reality of supply and demand curves in labor markets and concluded that the evidence is "at best inconclusive and at worst casts doubt on their existence." For instance, he cites Kaufman and Hotchkiss (2006): "For adult men, nearly all studies find the labour supply curve to be negatively sloped or backward bending." Supply and demand can be used to explain physician shortages, nursing shortages or teacher shortages.

In both classical and Keynesian economics, the money market is analyzed as a supply-and-demand system with interest rates being the price. The money supply may be a vertical supply curve, if the central bank of a country chooses to use monetary policy to fix its value regardless of the interest rate; in this case the money supply is totally inelastic. On the other hand, the money supply curve is a horizontal line if the central bank is targeting a fixed interest rate and ignoring the value of the money supply; in this case the money supply curve is perfectly elastic. The demand for money intersects with the money supply to determine the interest rate.

According to some studies, the laws of supply and demand are applicable not only to the business relationships of people, but to the behaviour of social animals and to all living things that interact on the biological markets in scarce resource environments.

The model of supply and demand accurately describes the characteristic of metabolic systems: specifically, it explains how feedback inhibition allows metabolic pathways to respond to the demand for a metabolic intermediates while minimizing effects due to variation in the supply.

Empirical estimation

Demand and supply relations in a market can be statistically estimated from price, quantity, and other data with sufficient information in the model. This can be done with simultaneous-equation methods of estimation in econometrics. Such methods allow solving for the model-relevant "structural coefficients," the estimated algebraic counterparts of the theory. The Parameter identification problem is a common issue in "structural estimation." Typically, data on exogenous variables (that is, variables other than price and quantity, both of which are endogenous variables) are needed to perform such an estimation. An alternative to "structural estimation" is reduced-form estimation, which regresses each of the endogenous variables on the respective exogenous variables.

Macroeconomic uses

Human-hours worked per week in the United States. Labor is supply, money is demand.

Demand and supply have also been generalized to explain macroeconomic variables in a market economy, including the quantity of total output and the aggregate price level. The aggregate demand-aggregate supply model may be the most direct application of supply and demand to macroeconomics, but other macroeconomic models also use supply and demand. Compared to microeconomic uses of demand and supply, different (and more controversial) theoretical considerations apply to such macroeconomic counterparts as aggregate demand and aggregate supply. Demand and supply are also used in macroeconomic theory to relate money supply and money demand to interest rates, and to relate labor supply and labor demand to wage rates.

History

The 256th couplet of Tirukkural, which was composed at least 2000 years ago, says that "if people do not consume a product or service, then there will not be anybody to supply that product or service for the sake of price".

According to Hamid S. Hosseini, the power of supply and demand was understood to some extent by several early Muslim scholars, such as fourteenth-century Syrian scholar Ibn Taymiyyah, who wrote: "If desire for goods increases while its availability decreases, its price rises. On the other hand, if availability of the good increases and the desire for it decreases, the price comes down."

If desire for goods increases while its availability decreases, its price rises. On the other hand, if availability of the good increases and the desire for it decreases, the price comes down.

— Ibn Taymiyyah,
Adam Smith

Shifting focus to the English etymology of the expression, it has been confirmed that the phrase 'supply and demand' was not used by English economics writers until after the end of the 17th century. In John Locke's 1691 work Some Considerations on the Consequences of the Lowering of Interest and the Raising of the Value of Money, Locke alluded to the idea of supply and demand, however, he failed to accurately label it as such and thus, he fell short in coining the phrase and conveying its true significance. Locke wrote: “The price of any commodity rises or falls by the proportion of the number of buyer and sellers” and “that which regulates the price... is nothing else but their quantity in proportion to Vent.” Locke's terminology drew criticism from John Law. Law argued that,"The Prices of Goods are not according to the quantity in proportion to the Vent, but in proportion to the Demand." From Law the demand part of the phrase was given its proper title and it began to circulate among "prominent authorities" in the 1730s. In 1755, Francis Hutcheson, in his A System of Moral Philosophy, furthered development toward the phrase by stipulating that, "the prices of goods depend on these two jointly, the Demand... and the Difficulty of acquiring."

It was not until 1767 that the phrase "supply and demand" was first used by Scottish writer James Denham-Steuart in his Inquiry into the Principles of Political Economy. He originated the use of this phrase by effectively combining "supply" and "demand" together in a number of different occasions such as price determination and competitive analysis. In Steuart's chapter entitled "Of Demand", he argues that "The nature of Demand is to encourage industry; and when it is regularly made, the effect of it is, that the supply for the most part is found to be in proportion to it, and then the demand is simple". It is presumably from this chapter that the idea spread to other authors and economic thinkers. Adam Smith used the phrase after Steuart in his 1776 book The Wealth of Nations. In The Wealth of Nations, Smith asserted that the supply price was fixed but that its "merit" (value) would decrease as its "scarcity" increased, this idea by Smith was later named the law of demand. In 1803, Thomas Robert Malthus used the phrase "supply and demand" twenty times in the second edition of the Essay on Population. And David Ricardo in his 1817 work, Principles of Political Economy and Taxation, titled one chapter, "On the Influence of Demand and Supply on Price". In Principles of Political Economy and Taxation, Ricardo more rigorously laid down the idea of the assumptions that were used to build his ideas of supply and demand. In 1838, Antoine Augustin Cournot developed a mathematical model of supply and demand in his Researches into the Mathematical Principles of Wealth, it included diagrams. It is important to note that the use of the phrase was still rare and only a few examples of more than 20 uses in a single work have been identified by the end of the second decade of the 19th century.

During the late 19th century the marginalist school of thought emerged. The main innovators of this approach were Stanley Jevons, Carl Menger, and Léon Walras. The key idea was that the price was set by the subjective value of a good at the margin. This was a substantial change from Adam Smith's thoughts on determining the supply price.

In his 1870 essay "On the Graphical Representation of Supply and Demand", Fleeming Jenkin in the course of "introduc the diagrammatic method into the English economic literature" published the first drawing of supply and demand curves in English, including comparative statics from a shift of supply or demand and application to the labor market. The model was further developed and popularized by Alfred Marshall in the 1890 textbook Principles of Economics.

Criticism

Piero Sraffa's critique focused on the inconsistency (except in implausible circumstances) of partial equilibrium analysis and the rationale for the upward slope of the supply curve in a market for a produced consumption good. The notability of Sraffa's critique is also demonstrated by Paul Samuelson's comments and engagements with it over many years, for example:

What a cleaned-up version of Sraffa (1926) establishes is how nearly empty are all of Marshall's partial equilibrium boxes. To a logical purist of Wittgenstein and Sraffa class, the Marshallian partial equilibrium box of constant cost is even more empty than the box of increasing cost.

Modern Post-Keynesians criticize the supply and demand model for failing to explain the prevalence of administered prices, in which retail prices are set by firms, primarily based on a mark-up over normal average unit costs, and are not responsive to changes in demand up to capacity.

See also

References

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