Missing Non-Voters and Misweighted Samples: Explaining the 2015 Great British Polling Miss

35 Pages Posted: 15 Jul 2015 Last revised: 13 Jan 2018

See all articles by Jonathan Mellon

Jonathan Mellon

West Point - Department of Systems Engineering

Christopher Prosser

University of London - Royal Holloway

Date Written: July 15, 2015

Abstract

The pre-election polls for the 2015 UK General Election missed the final result by a considerable margin: underestimating the Conservative Party and overestimating Labour. We analyse evidence for five theories of why the polls missed using British Election Study data. We find limited evidence for systematic vote intention misreporting, late swing, systematically different preferences among “don’t knows” or differential turnout of parties’ supporters. By comparing the BES face-to-face probability sample and online panel, we show that the online survey’s polling error is primarily caused by under-sampling non-voters, then weighting respondents to represent the general population. Consequently, demographic groups with a low probability of voting are over-weighted within the voter subsample. Finally, we show that this mechanism is likely partially responsible for the over-estimate of the Liberal Democrats in 2010, showing that this is a longstanding problem with British polls.

Keywords: polling miss; british election 2015; differential turnout; shy tories; late swing; don't know; representative samples; survey weighting; online polling; question ordering; survey methodology; political polling; public opinion

Suggested Citation

Mellon, Jonathan and Prosser, Christopher, Missing Non-Voters and Misweighted Samples: Explaining the 2015 Great British Polling Miss (July 15, 2015). Available at SSRN: https://ssrn.com/abstract=2631165 or http://dx.doi.org/10.2139/ssrn.2631165

Jonathan Mellon (Contact Author)

West Point - Department of Systems Engineering ( email )

600 Thayer Rd
West Point, NY 10996
United States

Christopher Prosser

University of London - Royal Holloway ( email )

Egham Hill
Egham, TW20 0EX

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