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An assessment of the causes of the errors in the 2015 UK General Election opinion polls

Sturgis, Patrick and Kuha, Jouni and Baker, Nick and Callegaro, Mario and Fisher, Stephen and Green, Jane and Jennings, Will and Lauderdale, Benjamin E. and Smith, Patten (2017) An assessment of the causes of the errors in the 2015 UK General Election opinion polls. Journal of the Royal Statistical Society. Series A: Statistics in Society. ISSN 0964-1998

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Identification Number: 10.1111/rssa.12329


The opinion polls undertaken prior to the 2015 UK General Election under-estimated the Conservative lead over Labour by an average of 7 percentage points. This collective failure led politicians and commentators to question the validity and utility of political polling and raised concerns regarding a broader public loss of confidence in survey research. In this paper, we assess the likely causes of the 2015 polling errors. We begin by setting out a formal account of the statistical methodology and assumptions required for valid estimation of party vote shares using quota sampling. We then describe the current approach of polling organisations for estimating sampling variability and suggest a new method based on bootstrap re-sampling. Next, we use poll micro-data to assess the plausibility of different explanations of the polling errors. Our conclusion is that the primary cause of the polling errors in 2015 was unrepresentative sampling.

Item Type: Article
Official URL:
Additional Information: © 2017 The Authors © CC BY 4.0
Subjects: J Political Science > JA Political science (General)
Sets: Departments > Methodology
Date Deposited: 04 Sep 2017 07:37
Last Modified: 29 Nov 2017 11:26

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