Doing More With Less: Mismatch-Based Risk-Limiting Audits
This work addresses election integrity by providing a more explainable and broadly applicable audit method, though it is incremental as it builds on existing RLA strategies with trade-offs in sample size.
The paper tackles the problem of risk-limiting audits (RLAs) for election verification by proposing a mismatch-based approach that simplifies auditing conceptually and extends applicability to more social choice functions, though it results in larger sample sizes, with increases being small under low mismatch rates and close margin bounds but potentially very large in other error scenarios.
One approach to risk-limiting audits (RLAs) compares randomly selected cast vote records (CVRs) to votes read by human auditors from the corresponding ballot cards. Historically, such methods reduce audit sample sizes by considering how each sampled CVR differs from the corresponding true vote, not merely whether they differ. Here we investigate the latter approach, auditing by testing whether the total number of mismatches in the full set of CVRs exceeds the minimum number of CVR errors required for the reported outcome to be wrong (the "CVR margin"). This strategy makes it possible to audit more social choice functions and simplifies RLAs conceptually, which makes it easier to explain than some other RLA approaches. The cost is larger sample sizes. "Mismatch-based RLAs" only require a lower bound on the CVR margin, which for some social choice functions is easier to calculate than the effect of particular errors. When the population rate of mismatches is low and the lower bound on the CVR margin is close to the true CVR margin, the increase in sample size is small. However, the increase may be very large when errors include errors that, if corrected, would widen the CVR margin rather than narrow it; errors affect the margin between candidates other than the reported winner with the fewest votes and the reported loser with the most votes; or errors that affect different margins.