The Twilight of the Polls? A Review of Trends in Polling Accuracy and the Causes of Polling Misses

Abstract

Polls have had a number of high profile misses in recent elections. We review the current polling environment, the performance of polls in a historical context, the mechanisms of polling error, and the causes of several high profile misses in Britain and the United States. Contrary to conventional wisdom, polling errors have been constant over time, although the level of error has always been substantially beyond that implied by stated margins of error. Generally there is little evidence that voters lying about their vote intention (so called Shy Voters) is a substantial cause of polling error. Instead polling errors have most commonly resulted from problems with representative samples and weighting, undecided voters breaking in one direction, and to a less extent correctly calibrating turnout models and late swing. We conclude with a discussion of future directions for polling both in terms of fixing the problem identified and new approaches to understanding public opinion.

Keywords: polling, survey error, non-probability samples, late swing, shy voters, turnout filtering, representative samples, survey weighting