April 29, 2011

Instant Runoff Voting Explained

The U.K. will be voting on the "Alternative Vote" next week. It is equivalent to what Americans call "Instant Run-off Voting": a fairer alternative to the "Winner Takes All" (U.S.) or "First Pass the Post" (U.K.) systems now in place.

The illustration below, adapted from the British campaign, illustrates why IRV is fairer.

How IRV works:
  • On the ballot, each voter marks first choice as "1", second choice (if any) as "2", and so on.
  • After vote, first choices are counted as usual.
  • If no candidate gets more than 50% of the first-choice votes, the candidate with the fewest first-choice votes is eliminated and the second choice on each of those ballots is counted (i.e., instant run-off).
  • This continues until one candidate gets more than 50% of the vote.