Computing the Margin of Victory in IRV Elections

Efficient post-election audits select the number of machines or precincts to audit based in part on the margin of victory (the number of ballots that must be changed in order to change the outcome); a close election needs more auditing than a landslide victory. For a simple “first-pastthe-post” election, the margin is easily computed based on the number of votes the first and second place candidates received. However, for instant runoff voting (IRV) elections, it is not immediately obvious how to compute the margin of victory. This paper presents algorithmic techniques for computing the margin of victory for IRV elections. We evaluate our method by attempting to compute the margin of victory for 25 IRV elections in the United States. The margin of victory computed can then be used to conduct post-election audits more effectively for IRV elections.