Instant Runoff Voting

Learning Outcomes

  • Determine the winner of an election using the Instant Runoff method
  • Evaluate the fairness of an Instant Runoff election

Study Strategy

When learning new vocabulary and processes it often takes more than a careful reading of the text to gain understanding. Remember to use flashcards for vocabulary, writing the answers out by hand before checking to see if you have them right. When learning new processes, writing them out by hand as you read through them will help you simultaneously memorize and gain insight into the process.

Pro-tip: Write out each of the examples in this section using paper and pencil, trying each of the steps as you go, until you feel you could explain it to another person.

Instant Runoff Voting

Instant Runoff Voting (IRV), also called Plurality with Elimination, is a modification of the plurality method that attempts to address the issue of insincere voting. In IRV, voting is done with preference ballots, and a preference schedule is generated. The choice with the least first-place votes is then eliminated from the election, and any votes for that candidate are redistributed to the voters’ next choice. This continues until a choice has a majority (over 50%).

This is similar to the idea of holding runoff elections, but since every voter’s order of preference is recorded on the ballot, the runoff can be computed without requiring a second costly election.

This voting method is used in several political elections around the world, including election of members of the Australian House of Representatives, and was used for county positions in Pierce County, Washington until it was eliminated by voters in 2009. A version of IRV is used by the International Olympic Committee to select host nations.

Example

Consider the preference schedule below, in which a company’s advertising team is voting on five different advertising slogans, called A, B, C, D, and E here for simplicity.

Initial votes

3 4 4 6 2 1
1st choice B C B D B E
2nd choice C A D C E A
3rd choice A D C A A D
4th choice D B A E C B
5th choice E E E B D C

If this was a plurality election, note that B would be the winner with 9 first-choice votes, compared to 6 for D, 4 for C, and 1 for E.

There are total of 3+4+4+6+2+1 = 20 votes. A majority would be 11 votes. No one yet has a majority, so we proceed to elimination rounds.

Round 1: We make our first elimination. Choice A has the fewest first-place votes, so we remove that choice

3 4 4 6 2 1
1st choice B C B D B E
2nd choice C D C E
3rd choice D C D
4th choice D B E C B
5th choice E E E B D C

We then shift everyone’s choices up to fill the gaps. There is still no choice with a majority, so we eliminate again.

3 4 4 6 2 1
1st choice B C B D B E
2nd choice C D D C E D
3rd choice D B C E C B
4th choice E E E B D C

Round 2: We make our second elimination. Choice E has the fewest first-place votes, so we remove that choice, shifting everyone’s options to fill the gaps.

3 4 4 6 2 1
1st choice B C B D B D
2nd choice C D D C C B
3rd choice D B C B D C

Notice that the first and fifth columns have the same preferences now, we can condense those down to one column.

5 4 4 6 1
1st choice B C B D D
2nd choice C D D C B
3rd choice D B C B C

Now B has 9 first-choice votes, C has 4 votes, and D has 7 votes. Still no majority, so we eliminate again.

Round 3: We make our third elimination. C has the fewest votes.

5 4 4 6 1
1st choice B D B D D
2nd choice D B D B B

Condensing this down:

9 11
1st choice B D
2nd choice D B

D has now gained a majority, and is declared the winner under IRV.

Try It

Consider again this election. Find the winner using IRV.

44 14 20 70 22 80 39
1st choice G G G M M B B
2nd choice M B G B M
3rd choice B M B G G

Here is an overview video that provides the definition of IRV, as well as an example of how to determine the winner of an election using IRV.

Please note: at 2:50 in the video it says 9+2+8=18, should 9+2+8=19, so D=19

What’s Wrong with IRV?

Example

Let’s return to our City Council Election.

342 214 298
1st choice Elle Don Key
2nd choice Don Key Don
3rd choice Key Elle Elle

In this election, Don has the smallest number of first place votes, so Don is eliminated in the first round. The 214 people who voted for Don have their votes transferred to their second choice, Key.

342 512
1st choice Elle Key
2nd choice Key Elle

So Key is the winner under the IRV method.

We can immediately notice that in this election, IRV violates the Condorcet Criterion, since we determined earlier that Don was the Condorcet winner. On the other hand, the temptation has been removed for Don’s supporters to vote for Key; they now know their vote will be transferred to Key, not simply discarded.

In the following video, we provide the example from above where we find that the IRV method violates the Condorcet Criterion in an election for a city council seat.

Example

Consider the voting system below.

37 22 12 29
1st choice Adams Brown Brown Carter
2nd choice Brown Carter Adams Adams
3rd choice Carter Adams Carter Brown

In this election, Carter would be eliminated in the first round, and Adams would be the winner with 66 votes to 34 for Brown.

Now suppose that the results were announced, but election officials accidentally destroyed the ballots before they could be certified, and the votes had to be recast. Wanting to “jump on the bandwagon,” 10 of the voters who had originally voted in the order Brown, Adams, Carter change their vote to favor the presumed winner, changing those votes to Adams, Brown, Carter.

47 22 2 29
1st choice Adams Brown Brown Carter
2nd choice Brown Carter Adams Adams
3rd choice Carter Adams Carter Brown

In this re-vote, Brown will be eliminated in the first round, having the fewest first-place votes. After transferring votes, we find that Carter will win this election with 51 votes to Adams’ 49 votes! Even though the only vote changes made favored Adams, the change ended up costing Adams the election. This doesn’t seem right, and introduces our second fairness criterion:

Monotonicity Criterion

If voters change their votes to increase the preference for a candidate, it should not harm that candidate’s chances of winning.

This criterion is violated by this election. Note that even though the criterion is violated in this particular election, it does not mean that IRV always violates the criterion; just that IRV has the potential to violate the criterion in certain elections.

The last video shows the example from above where the monotonicity criterion is violated.