An Assessment of the Causes of the Errors in the 2015 UK General Election Opinion Polls

Patrick Sturgis, Jouni Kuha, Nick Baker, Mario Callegaro, Stephen Fisher, Jane Green, Will Jennings, Benjamin Lauderdale, Patten Smith

Research output: Contribution to journalArticlepeer-review


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.
Original languageEnglish
Article numberRSSA12329
JournalRoyal Statistical Society. Journal. Series A: Statistics in Society
Early online date25 Oct 2017
Publication statusPublished - 2017

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  • Cathie Marsh Institute


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