American Statistical Association Statement on Risk-Limiting Post-Election Audits
Poorly marked ballots, computer glitches, and voting system configuration errors can make machine vote counts diverge from voters' intentions. By comparing hand counts of randomly selected ballots with machine tallies, we can judge whether a full hand count would show the same winners. Such audits can improve trust in our elections. Statisticians can help design efficient audits that save taxpayers' money and election officials’ time.
Many states now require auditing a fixed percentage of all ballots. While any auditing is good, fixed- percentage audits frequently select more ballots than needed – and sometimes not enough – to provide strong evidence that the machine-count1 winner(s) truly received the most votes. In contrast,
"risk-limiting" audits are designed to always have an acceptably small probability of failing to correct a wrong machine-count outcome. When a machine-count outcome is correct, a risk-limiting audit can often confirm the result after examining only a small fraction of the ballots cast.
The American Statistical Association (ASA) recommends that risk-limiting audits be routinely conducted and reported in all federal, most state-wide, and at least a sampling of other governmental election contests. The ASA urges state and local officials to seek statistical advice on how to sample and analyze data to efficiently attain the desired level of risk control. This statement builds upon previous actions of ASA’s Board of Directors. In March 2008, the Board issued a position statement2 on election integrity, saying (in part) that “... the integrity of central vote tabulations [must] be confirmed by audits of voter-verified hardcopy records … to provide high – and clearly specified – levels of confidence in electoral outcomes.” In August 2008, the Board endorsed the statistics-relevant parts of Principles and Best Practices for Post-Election Audits.3
Risk-Limiting Audits and Sampling
Post-election audits generally rely on random sampling of batches of ballots, with requirements:
• Sampling occurs after machine-count batch totals have been reported
• Batches and ballots to be selected are not known in advance
• All ballots are taken into account
To be risk-limiting, the overall procedure must ensure that if the machine-count electoral outcome is incorrect, there is a large, pre-specified chance that the audit will reveal the correct outcome. For further discussion and requirements, see Principles and Best Practices for Post-Election Audits.2
Credible audits are not possible if votes are cast and counted only electronically because there is no original record of voter intent with which to compare. An audit based on examining a sample of paper records is a good check on the machine tallies because the two ways of counting are vulnerable
to different kinds of errors.
Checking a non-random sample only tells us about these particular ballots; it says nothing about the remaining ones. In contrast, the errors found in a random sample allow inferences about errors in not-yet-examined ballot batches. Statistics provides tools for evaluating the strength of the evidence that
an outcome is correct despite errors found.
The risk we seek to limit is the chance of certifying the machine-count outcome of any contest for which a full hand count would yield a different outcome. Statistics can be used to design audits that
1 The machine-count is the outcome that will become official unless a full hand count determines it is wrong.
For a PDF version of this statement: