CAST: Canvass Audits by Sampling and Testing

Abstract

Canvass audit by sampling and testing (CAST) is a method to decide on the basis of a hand count of a random sample of batches of ballots whether to count the entire audit trail of a contest by hand. Data are collected in stages. At stage s, one computes Ps , an upper bound on the P-value of the hypothesis that the apparent outcome is wrong, conditional on the results of previous audit stages. If Ps is less than a threshold alphas-strong evidence that the apparent outcome is correct-the audit stops. Otherwise, the audit advances to stage s+1 . If the audit has not stopped by stage s=S, there is a full hand count. The thresholds {alphas}s=1S are chosen to guarantee that whenever the apparent outcome is wrong, the chance of a full hand count is at least 1-alpha: The risk of certifying an incorrect outcome is at most alpha. Conditioning allows deliberate selection of some batches of ballots for audit. CAST can use any sampling and testing scheme for which upper bounds on conditional P -values can be calculated. A turn-key implementation for proportionally stratified random samples is given, using the maximum relative overstatement of pairwise margins as the test statistic. This implementation works with contests that cross jurisdictional boundaries and contests that allow voters to select more than one candidate.