# Next Steps for the Colorado Risk-Limiting Audit (CORLA) Program

## TL;DR

This paper proposes new, more efficient methods for conducting risk-limiting audits across multiple counties, combining ballot polling and comparison techniques, with practical examples and open-source tools.

## Contribution

It introduces novel methods for cross-county risk-limiting audits that integrate ballot polling and comparison, improving efficiency over previous approaches.

## Key findings

- Stratified sampling methods improve audit efficiency.
- Combining ballot polling and comparison reduces audit workload.
- Open-source tools facilitate implementation and exploration.

## 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.