Bottom-Two-Runoff IRV

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Revision as of 19:15, 9 February 2019 by Voting-Stuff (talk | contribs) (Copied the Tennessee Capital Election example from the IRV page. Will now adapt.)
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Bottom-Two-Runoff IRV (BTR-IRV) is a voting system that selects a single winner using votes that express ranked preferences. BTR-IRV passes the Condorcet Criterion, and is by definition a Condorcet method.

BTR-IRV was conceived as a sort of Instant-runoff voting-Condorcet hybrid, in that the runoff it uses is very similar to IRV, but the runoff doesn't ever eliminate a Condorcet Winner.

The process is as follows: take the two options with the fewest first preference votes. The pairwise loser out of those two options is eliminated, and the next preferences from those ballots are redistributed. This process repeats until there is only one option remaining, and that remaining option is the winner.

It was originally proposed by Rob LeGrand.[1][2]

An example (In Progress)

Tennessee's four cities are spread throughout the state

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)
  1. Memphis
  2. Nashville
  3. Chattanooga
  4. Knoxville
  1. Nashville
  2. Chattanooga
  3. Knoxville
  4. Memphis
  1. Chattanooga
  2. Knoxville
  3. Nashville
  4. Memphis
  1. Knoxville
  2. Chattanooga
  3. Nashville
  4. Memphis
City Round 1 Round 2 Round 3
Memphis 42 42 42
Nashville 26 26 0
Chattanooga 15 0 0
Knoxville 17 32 58

Chattanooga, having the smallest vote, is eliminated in the first round. All of the votes for Chattanooga have Knoxville as a second choice, so they are transferred to Knoxville. Nashville now has the smallest vote, so it is eliminated. The votes for Nashville have Chattanooga as a second choice, but as Chattanooga has been eliminated, they instead transfer to their third choice, Knoxville. Knoxville now has 58% of the vote, and it is the winner.

In a real election, of course, voters would show greater variation in the rankings they cast, which could influence the result.

External links