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

35 Pages Posted: 15 Jul 2015 Last revised: 10 Nov 2016

Jonathan Mellon

University of Oxford - Nuffield College; University of Manchester

Christopher Prosser

University of Manchester

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)

University of Oxford - Nuffield College ( email )

New Road
Oxford, OX1 1NF
United Kingdom

University of Manchester ( email )

Oxford Road
Manchester, M13 9PL
United Kingdom

Christopher Prosser

University of Manchester ( email )

Oxford Road
Manchester, M13 9PL
United Kingdom

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