Election polling errors across time and space

Are election polling misses becoming more prevalent? Are they more likely in some contexts than others? Here we undertake an over-time and cross-national assessment of prediction errors in pre-election polls. Our analysis draws on more than 30,000 national polls from 351 general elections in 45 countries between 1942 and 2017. We proceed in the following way. First, building on previous studies, we show how errors in national polls evolve in a structured way over the election timeline. Second, we examine errors in polls in the final week of the election campaign to assess performance across election years. Third, we undertake a pooled analysis of polling errors—controlling for a number of institutional and party features—that enables us to test whether poll errors have increased or decreased over time. We find that, contrary to conventional wisdom, the recent performance of polls has not been outside the ordinary. However, the performance of polls does vary across political contexts and in understandable ways.