Untangling the Dueling Expert Witnesses: Comparing Ensemble Methods in Pennsylvania's Redistricting Plans

Ensembles of random legislative districts are a valuable tool for assessing whether a proposed district plan is an outlier or gerrymander. Expert witnesses have presented these in litigation using various methods, and unsurprisingly, they often disagree. Recent open source methods now permit independent validation of expert witness testimony. Here, we compare ensembles for the Pennsylvania House and Congressional districts calculated using "Redist" and "Gerrychain" further incorporating constraints restricting county and municipal boundary splitting, as required by Pennsylvania for legal plans. We compare results to expert witness testimony submitted by Republican and Democratic parties. We confirm some of the testimony but could not reproduce all of it, struggling with metrics based on a heuristic "sum of votes index" rathern than a straightforward average of metrics across multiple elections. We recommend against relying on analytics based on summing votes from multiple elections to create vote incides and derivative metrics as these are inherently poorly behaved. To promote transparency, we recommend that where possible, expert witness testimony be based solely on publicly available election data as opposed to proprietary data closely held by political parties.