Researchers are increasingly reliant on online, opt-in surveys. But prior benchmarking exercises employ national samples, making it unclear whether such surveys can effectively represent Black respondents and other minorities nationwide. This paper presents the results of uncompensated online and in-person surveys administered chiefly in one racially diverse American city—Philadelphia—during its 2023 mayoral primary. The participation rate for online surveys promoted via Facebook and Instagram was .4%, with White residents and those with college degrees more likely to respond. Such biases help explain why neither our surveys nor public polls correctly identified the Democratic primary’s winner, an establishment-backed Black Democrat. Even weighted, geographically stratified online surveys typically underestimate the winner’s support, although an in-person exit poll does not. We identify some similar patterns in Chicago. These results indicate important gaps in the populations represented in contemporary opt-in surveys and suggest that alternative survey modes help reduce them.
