Limiting Risk by Turning Manifest Phantoms into Evil Zombies

Drawing a random sample of ballots to conduct a risklimiting audit generally requires knowing how the ballots cast in an election are organized into groups, for instance, how many containers of ballots there are in all and how many ballots are in each container. A list of the ballot group identifiers along with number of ballots in each group is called a ballot manifest. What if the ballot manifest is not accurate? We present a simple modification to some risk-limiting audit procedures that makes them automatically become more conservative if the ballot manifest has errors. The modification—phantoms to evil zombies ( 2EZ)—requires only an upper bound on the total number of ballots cast. 2EZ makes the audit P-value stochastically larger than it would be had the manifest been accurate, automatically requiring more than enough ballots to be audited to offset the manifest errors. This ensures that the true risk limit remains smaller than the nominal risk limit. On the other hand, if the manifest is in fact accurate and the upper bound on the total number of ballots equals the total according to the manifest, 2EZ has no effect at all on the number of ballots audited nor on the true risk limit.