Instant-Runoff Voting with Bottom Two Runoff
Instant-Runoff Voting with Bottom Two Runoff (abbreviated as IRV-BTR) is an election vote-counting method that uses ranked ballots. In each round the two candidates with the fewest first place votes are put in a simulated runoff where it is determined which of the two would lose in a head-to-head election using the rankings from all ballots.
This method modifies instant runoff voting (IRV) by adding a runoff between the bottom two candidates and can be viewed as an expansion of RCIPE by providing a broader criteria for when the voter with the fewest first rank votes is not eliminated. This addition makes it satisfy the condorcet criterion and would have prevented the failure of instant-runoff voting to elect the most popular candidate in the 2009 mayoral election in Burlington, Vermont.
Description
Voters rank the candidates using as many ranking levels as there are candidates, or at least five ranking levels. Using fewer ranking levels than candidates is useful on paper ballots where space is limited, and in elections where there are numerous candidates who are unlikely to win.
This method eliminates one candidate at a time. The final remaining (not-yet-eliminated) candidate is declared to be the winner.
In each round the two candidates are put into a simulated runoff. Each of the candidates receives a simulated vote for every ballot on which they are ranked higher than their opponent. The candidate with the fewer simulated votes is eliminated.
The last candidate to be eliminated is the runner-up candidate. If this counting method is used in the primary election of a major political party, and if the runoff or "general" election is counted in a way that is not vulnerable to vote splitting, then ideally the runner-up candidate would move to the runoff or general election along with the primary-election winner. Small political parties would not qualify to move their runner-up candidate to the runoff or general election.
Importantly, the runner-up candidate does not deserve to win any kind of elected seat. This means this method is not suitable for filling multiple seats, such as on a city council or in a multi-member district.
To avoid spoiled ballots in elections where the voter uses a pen or marker to mark their paper ballot, more than one candidate can be marked at the same ranking level. Such ballots would count for neither candidate in the case of a simulated runoff between the two.
Also to avoid spoiled ballots, if a voter marks more than one ranking level for the same candidate, only the highest-marked ranking level is used during counting.
If the voter does not mark any ovals for a candidate, that candidate is ranked at the lowest ranking level, as if the voter marked the oval for the lowest ranking level.
The ranking level below the lowest ranking level is reserved for write-in candidates whose names do not appear on the ballot being counted.
Example
Imagine that Tennessee is having an election on the location of its capital. The population of Tennessee is concentrated around its four major cities, which are spread throughout the state. For this example, suppose that the entire electorate lives in these four cities, and that everyone wants to live as near the capital as possible.
The candidates for the capital are:
- Memphis, the state's largest city, with 42% of the voters, but located far from the other cities
- Nashville, with 26% of the voters, near the center of Tennessee
- Knoxville, with 17% of the voters
- Chattanooga, with 15% of the voters
The preferences of the voters would be divided like this:
| 42% of voters (close to Memphis) |
26% of voters (close to Nashville) |
15% of voters (close to Chattanooga) |
17% of voters (close to Knoxville) |
|---|---|---|---|
|
|
|
|
These ballot preferences are converted into pairwise counts and displayed in the following tally table.
| All possible pairs of choice names |
Number of votes with indicated preference | ||
|---|---|---|---|
| Prefer X over Y | Equal preference | Prefer Y over X | |
| X = Memphis Y = Nashville |
42% | 0 | 58% |
| X = Memphis Y = Chattanooga |
42% | 0 | 58% |
| X = Memphis Y = Knoxville |
42% | 0 | 58% |
| X = Nashville Y = Chattanooga |
68% | 0 | 32% |
| X = Nashville Y = Knoxville |
68% | 0 | 32% |
| X = Chattanooga Y = Knoxville |
83% | 0 | 17% |
The initial rankings are 42% Memphis, 26% Nashville, 17% Knoxville, and 15% Chattanooga.
In the first elimination round Knoxville and Chattanooga are compared to each other. Knoxville is eliminated because it loses the pairwise comparison to Chattanooga (83%-17%).
If there had not been a runoff, Chattanooga would have been eliminated because it has the smallest number of ballots that rank Chattanooga as the first choice (15% - 17%).
The rankings would now be 42% Memphis, 32% Chattanooga, and 26% Nashville.
In the second elimination round, Chattanooga and Nashville are compared. In this case Chattanooga is eliminated because it loses the pairwise comparison (32%-68%). Nashville would have been eliminated in a normal IRV election due to it having fewer first rank votes.
In the third and final elimination round, Memphis is eliminated by Nashville (42% - 58%). This final round is indistinguishable from a standard IRV final round due to the existence of only two candidates.
Mathematical criteria
The frequency with which this method passes or fails each of the following criteria have not been estimated.
This method always passes the following criteria.
- Condorcet: pass
- Condorcet loser: pass
- Smith/ISDA: pass
The below methods have not been evaluated at this point.
- Resolvable: pass
- Polytime: pass
- Majority: fail
- Majority loser: fail
- Mutual majority: fail
- LIIA: fail
- IIA: fail
- Cloneproof: fail
- Monotone: fail
- Consistency: fail
- Reversal symmetry: fail
- Later no harm: fail
- Later no help: fail
- Burying: fail
- Participation: fail
- No favorite betrayal: fail
It is summable with O(N2).