@Unpublished{Riv07b,
author = { Ronald L. Rivest },
title = { A Simple Rule of Thumb for Election Audit Size Determination },
note = { Unpublished draft. },
date = { 2007-10-31 },
OPTyear = { 2007 },
OPTmonth = { October 31, },
abstract = {
This note proposes a very simple ``Rule of Thumb'' for
determining how many precincts to audit in a post-election audit.
This rule says to audit $1/m$
precincts (rounded up to the next integer), where $m$ is
the ``margin of victory'' (the difference in percentage
of votes for the apparent winner and for the apparent
runner-up). For example, if the margin of victory is
4\%, then this rule of thumb suggests auditing
$ 1.0/0.04 = 25$ precincts. If the formula suggests auditing more
precincts than are available, all precincts are audited.
\par
This Rule of Thumb gives a guaranteed ``confidence
level'' of at least 92\% when certain assumptions hold
(when all precincts have the same size and the miscount
within each precinct is at most 20\% of the votes in that
precinct). Under these assumptions, the actual confidence
level achieved ranges from 92\% to 100\%.
\par
This Rule of Thumb is simple enough to be computed
in one's head; it provides reasonable guidance for
``back-of-the-envelope'' estimates, and is useful
when the simplicity of the formula is especially important.
},
}