Workload Estimates for Risk-Limiting Audits of Large Contests

We compare the expected number of ballots that must be counted by hand for two risk-limiting auditing methods, Canvass Audits by Sampling and Testing (CAST) and Kaplan-Markov (KM). The methods use different sampling designs to select batches of ballots to count by hand and different test statistics to decide when the audit can stop. The comparisons are based on the 2008 U.S. House of Representatives contests in California. The comparisons include hypothetical errors in the precinct vote totals, but errors are assumed to be small enough that the electoral outcomes are still correct. KM requires auditing fewer ballots than CAST. The workload for CAST can be reduced modestly by optimizing the number of precincts drawn from each county. Stratification by county is necessary for the practical implementation of risk-limiting audit methods in cross-jurisdictional contests. Workload can be reduced substantially, for both KM and CAST, by tallying ballots in batches smaller than precincts: Workload is roughly proportional to the average size of the batches. We discuss several methods to reduce batch sizes using current vote tabulation systems.