Lazy Risk-Limiting Ballot Comparison Audits
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Risk-limiting audits or RLAs are rigorous statistical procedures meant to detect invalid election results. In general, they call for an examination of paper ballots cast during the election to statistically assess potential disagreements between the winner determined by the ballots and the winner reported by tabulation. The design of an RLA must balance risk, the chance that the audit fails to detect such a disagreement when one occurs, with efficiency, the total effort to prepare and conduct the audit. The most efficient approaches-when measured in terms of the number of ballots that must be inspected-proceed by ballot comparison. This technique, however, requires an untrusted declaration of the contents of each cast ballot, rather than a simple tabulation of vote totals. This cast-vote record (CVR) is then spot-checked against ballots for consistency. The cost of generating such a CVR dominates the cost to conduct the audit in many practical settings which precludes adoption. We introduce a new RLA procedure: a lazy ballot comparison audit. In this audit, a global CVR is never produced; instead, when a ballot is selected for audit a three-stage procedure is followed: 1) the batch containing the ballot is announced, 2) a CVR is produced for that batch, and 3) the ballot is announced and compared with the CVR. The analysis of such an audit is complicated by the fact that the details of each CVR may depend-perhaps adversarially-on the prior history of the audit. We present three main contributions: (1) A formal adversarial model for RLAs; (2) Definition and analysis of a lazy audit procedure with rigorous risk limits and an associated correctness analysis accounting for the incidental errors arising in typical audits; and (3) An analysis of the relative time savings of these techniques.
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