Title: Next Steps for the Colorado Risk-Limiting Audit (CORLA) Program,
Abstract: Colorado conducted risk-limiting tabulation audits (RLAs) across the state in
2017, including both ballot-level comparison audits and ballot-polling audits.
Those audits only covered contests restricted to a single county; methods to
efficiently audit contests that cross county boundaries and combine ballot
polling and ballot-level comparisons have not been available.
Colorado's current audit software (RLATool) needs to be improved to audit
these contests that cross county lines and to audit small contests efficiently.
This paper addresses these needs. It presents extremely simple but
inefficient methods, more efficient methods that combine ballot polling and
ballot-level comparisons using stratified samples, and methods that combine
ballot-level comparison and variable-size batch comparison audits in a way that
does not require stratified sampling.
We conclude with some recommendations, and illustrate our recommended method
using examples that compare them to existing approaches. Exemplar open-source
code and interactive Jupyter notebooks are provided that implement the methods
and allow further exploration.