| Ranking | # Voters | |
|---|---|---|
| 1. | 2. | |
| Total: | ||
Candidate Selectors
Instructions
This interactive demo is designed to illustrate how IRV works as well as its pros and cons. Each row in the table represents a possible ballot permutation and how many people voted that way. Change the vote counts for each ballot permutation, then click VOTE to view the results. Candidates can also be deselected to observe how the election would play out, had they not been an option.
Have real-world data, or a specific example? A ballot upload option is in the works. Feel free to suggest a ballot format for the future uploads.
Information / Examples / Voting Method Criteria
How it works
Under Instant Runoff Voting, voters rank candidates in order of preference. During the vote tallying, only the first preferences from each ballot are counted. If a candidate receives 50% or more of the vote, they win. If no candidate receives 50% of the vote, the lowest ranking candidate is eliminated and the votes are recounted, ignoring the eliminated candidate. This continues until a candidate has reached 50% of the vote.[1]
Monotonicity
Ranking or rating a candidate higher should never cause that candidate to lose, nor should ranking or rating a candidate lower ever cause that candidate to win, assuming all other candidates remain rated or ranked the same.
[2] Surprisingly, IRV fails to satisfy this criterion. This can be seen in the 2009 Burlington VT mayorial election.[3] The original ballot totals can be entered by clicking here. In this election the candidates are: Bob Kiss (A), Andy Montroll (B), and Kurt Wright (C). In the real election, Montroll (B) is eliminated in the first round, and Kiss (A) beats Wright (C) in the final round. If all 495 of the C > A > B voters and 258 of the C-only voters had instead voted for A, then C would have been eliminated in the first round, and A would have lost to B in the final round. These hypothetical vote changes can be entered by clicking here. So what is the significance of this? This means that by increasing their support for Mr. Kiss (A), those voters caused him to lose.
Order of Elimination
Under IRV, the order of elimination, that is, the order in which candidates are eliminated, can affect the election outcome. Why is this, and does it matter? To illustrate why/how it happens, click here to load a set of example ballots. In this example, the elimination of A first is compared with B first. A-voters prefer C to B/D, and B-voters prefer D to A/C. Naturally, when one is eliminated, that bloc's next preference is strengthened, in this example, going on to win. To switch from the A-case to the B-case, decrease ballot "BA" by 1, and increase ballot "AB" by 1. Should an election be swayed/decided by a voter who has gave no preference on the two "stronger" candidates (C/D)? This shows how unpredictable IRV can be.
The Spoiler Effect
A spoiler is a non-winning candidate whose presence on the ballot affects which candidate wins.
[4] A common argument among proponents of IRV is that it eliminates the spoiler effect,[5] but in reality, the effect is only mitigated by eliminating weak candidates. The spoilers that do occur are hidden by the multi-round tallying process. Discovering a spoiler in a real-world election would require access to the ballots. An example of the spoiler effect can be seen in this exmaple: click here to enter the vote totals. Simulating the election with all three candidates, C wins. However, removing candidate B and resimulating the election, A wins. This means that candidate B spoiled the election for candidate A. Other examples can be found by regenerating the ballots, removing a losing candidate and checking for a change in the winner. IRV appears to eliminate spoilers, in part, because it appears to prevent vote splitting by allowing secondary preferences.
Tactical Voting[6]
Proponents of IRV claim it is highly resistant to tactical voting.[7] The idea is that due to candidate ranking on the ballot, it is safe for voters to rank their favorite candidate first, second favorite second, and so on. This claim is subject to debate, as some claim that it can be better for voters to not rank their favorite first.[6] Others claim it reduces tactical voting compared to Simple Plurality (FPTP), but is not immune to it.[8]