Measuring lost votes by mail

The rise of mail balloting has led to concerns that procedural requirements can lead to "lost votes by mail." We theorize how procedural requirements can affect the incidence and form of lost votes and highlight three measurement issues with equating lost votes and rejected mail ballots. First, coverage: Not all rejected mail ballots are documented. Second, substitution: Some people whose mail ballot is rejected may subsequently successfully vote, in particular if they were notified in time to take action. Third, deterrence: Others may not return their mail ballots if they expect them to be rejected. While rejected mail ballots could over- or underestimate lost votes, a case study of Pennsylvania's 2022 general election reveals at least 47% more lost votes than rejected mail ballots. These lost votes could prove electorally consequential in Pennsylvania given the number of mail ballots cast and the substantial partisan splits on mail versus in-person ballots.