  Drawing a random sample of ballots to conduct a risk-limiting 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? Surprisingly, even if ballots are known to be
missing from the manifest, it is not necessary to make worst-case assumptions
about those ballots--for instance, to adjust the margin by the number of
missing ballots--to ensure that the audit remains conservative. Rather, it
suffices to make worst-case assumptions about the individual randomly selected
ballots that the audit cannot find. This observation provides 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.
