Election polling errors across time and space

Abstract

Are election polling misses becoming more prevalent? Are they more likely in some contexts than others? In this paper we undertake an over-time and cross-national assessment of prediction errors in pre-election polls. Our analysis draws on more than 26,000 polls from 338 elections in 45 countries over the period between 1942 and 2013, as well as data on more recent elections from 2014 to 2016. We proceed in the following way. First, building on previous studies, we demonstrate how poll errors evolve in a structured way over the election timeline. Second, we then focus on errors in polls in the final week of the campaign to examine poll performance across election years. Third, we use the historical performance of polls to benchmark recent polling “misses” in the UK, US and elsewhere. Fourth, we undertake a pooled analysis of polling errors – controlling for a number of institutional and party features – which enables us to test whether poll errors have increased or decreased over time. We find that, contrary to conventional wisdom, recent performance of polls has not been outside the ordinary. The performance of polls does vary across political contexts, however, in understandable ways.

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ORCID for William Jennings: orcid.org/0000-0001-9007-8896

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