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Instant-runoff voting

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Instant Runoff Voting (known as Alternative Vote in many countries) is a Voting system for single-seat elections. Mathematically, it is simply Single transfer voting with a district size of 1. It is designed to emulate a series of runoff elections.


This system encourages popularity over acceptability: any bloc of more

than half the voters can elect a candidate regardless of the opinion

of the rest of the voters. Candidates must also receive enough

initial first-place votes to stay involved in the election, even if

they have a large number of second- or third- place votes.


Voting

Each voter ranks all candidates in order of preference.


Counting The Votes

Top-preference votes are tallied. The candidate with the least

support is eliminated, and their votes are reallocated to the

next-highest ranked candidates on the eliminated ballots. After a

candidate is eliminated, they may not receive any more votes.


This process is repeated until one candidate has received fifty

percent of the votes cast.


An example =

Four candidates: Andrea, Brad, Carter, and Delilah.


12 voters rank the candidates:


  1. Andrea
  1. Brad
  1. Carter
  1. Delilah


8 voters rank the candidates:


  1. Carter
  1. Brad
  1. Delilah
  1. Andrea


4 voters rank the candidates:


  1. Delilah
  1. Brad
  1. Carter
  1. Andrea


1 voter ranked the candidats:


  1. Brad
  1. Carter
  1. Andrea
  1. Delilah


As none of the candidates have reached 50%, the lowest-ranked

candidate, Brad, is eliminated. The one vote for him is transferred

to Carter. The vote table now stands:


Andrea: 13

Carter: 9

Delilah: 4


Delilah is eliminated. The four votes she received are transferred to

the next eligible candidate on the ballots that voted for her, Carter.

(Brad has been eliminated.) Carter has now received 13 votes and he

is elected.


Potential for Tactical Voting

Tactical voting is useless in IRV. If you vote for a candidate

who does not have a good chance to win, you still get to express a

preference between the most-popular candidates. No advantage is given

to those voters who expressed their preference for a popular candidate

earlier, rather than later.


See also: Runoff voting