  The margin of victory is easy to compute for many election schemes but
difficult for Instant Runoff Voting (IRV). This is important because arguments
about the correctness of an election outcome usually rely on the size of the
electoral margin. For example, risk-limiting audits require a knowledge of the
margin of victory in order to determine how much auditing is necessary. This
paper presents a practical branch-and-bound algorithm for exact IRV margin
computation that substantially improves on the current best-known approach.
Although exponential in the worst case, our algorithm runs efficiently in
practice on all the real examples we could find. We can efficiently discover
exact margins on election instances that cannot be solved by the current
state-of-the-art.
