A risk-limiting audit (RLA) is a post-election tabulation auditing procedure which can limit the risk that the reported outcome in an election contest is incorrect. It generally involves (1) storing voter-verified paper ballots securely until they can be checked,[1] and (2) manually examining a statistical sample of the paper ballots until enough evidence is gathered to meet the risk limit.[2][3]
Advantages of an RLA include: samples can be small and inexpensive if the margin of victory is large;[2] there are options for the public to watch and verify each step;[1] and errors found in any step lead to corrective actions, including larger samples, up to a 100% hand count if needed. Disadvantages include: the sample needs to be a large fraction of all ballots to minimize the chance of missing mistakes, if any contest is close; and it is hard to check computer totals publicly, except by releasing computer records to the public.[4] If examining sampled ballots shows flaws in ballot storage, the usual approach cannot recover correct results,[5] and researchers recommend a re-vote if the number of ballots held in flawed storage is enough to change winners.[6] An alternative to re-votes is to create and verify backups of the paper ballots soon after they are voted, so there is an alternative to flawed storage of the original ballots.
As with other election audits, the goal is to identify not only intentional alterations of ballots and tallies, but also bugs in election machines, such as software errors, scanners with blocked sensors[7] or scanners skipping some ballots. The approach does not assume that all ballots, contests or machines were handled the same way, in which case spot checks could suffice. The sample sizes are designed to have a high chance of catching even a brief period when a scratch or fleck of paper blocks one sensor of one scanner, or a bug or hack switches votes in one precinct or one contest, if these problems affect enough ballots to change the result.
Comparisons can be done ballot-by-ballot or precinct-by-precinct, though the latter is more expensive.[8]
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