Replication Data for: Electoral predictors of polling errors

Abstract

Case studies of polling failures focus on within-election differences in poll accuracy. The crucial question of why polls fail in one election but not in others often remains a matter of speculation. To develop a contextual understanding, we review and unify the- ories of election features suspected of encouraging polling errors, including mobilization, candidacies, polarization, and electoral conduct. We extend a Bayesian hierarchical modeling approach that separates poll bias and variance at the election level and links error components to electoral predictors. Investigating 6,375 pre-election polls across 318 U.S. Senate elections, 1990-2022, we find an overall trend toward smaller but more uniform errors. Poll variance exhibits a weak negative association with mobilization and polarization. Until 2004, frontrunners and incumbents were overestimated, but there is little evidence that polls are biased for female or minority candidates. Finally, Republicans in states with lower levels of state democracy are slightly underestimated in recent years.