PROVIDENCE: a Flexible Round-by-Round Risk-Limiting Audit

A Risk-Limiting Audit (RLA) is a statistical election tabulation audit with a rigorous error guarantee. We present ballot polling RLA PROVIDENCE, an audit with the efficiency of MINERVA and flexibility of BRAVO, and prove that it is risk-limiting in the presence of an adversary who can choose subsequent round sizes given knowledge of previous samples. We describe a measure of audit workload as a function of the number of rounds, precincts touched, and ballots drawn and quantify the problem of obtaining a misleading audit sample when rounds are too small, demonstrating the importance of the resulting constraint on audit planning. We describe an approach to planning audit round schedules using these measures and present simulation results demonstrating the superiority of PROVIDENCE. We describe the use of PROVIDENCE by the Rhode Island Board of Elections in a tabulation audit of the 2021 election. Our implementation of PROVIDENCE in the open source R2B2 library has been integrated as an option in Arlo, the most commonly used RLA software.