Risk-Limiting Vote-Tabulation Audits: The Importance of Cluster Size

Post-election vote-tabulation audits compare hand counts of votes in a collection of groups (clusters) of paper records ("voter-verified paper audit trail," VVPAT) to reported machine counts of the votes in the same clusters. Votetabulation audits can serve a variety of roles, including process monitoring, quality improvement, fraud deterrence, and bolstering public confidence. All of these raise statistical issues. This note focuses on audits that check whether the machine-count outcome