Hartman, T.K.

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16/08/2017 GE2017: Can you trust that surprise exit poll?https://theconversation.com/ge2017-can-you-trust-that-surprise-exit-poll-79170 1/4AuthorTodd K. HartmanLecturer in Quantitative Social Science,Sheffield Methods Institute, University ofSheffieldAcademic rigour, journalistic flairTrue to form, the 2017 UK election exit poll has brought shock to election night as theprospect of the Tories failing to secure a majority in parliament appears to be a realpossibility. The poll puts the Conservatives on 314 seats – 12 short of a majority –and Labour on 266 seats.PA/ Andrew MatthewJune 9, 2017 12.35am BSTGE2017: Can you trust that surprise exit poll?16/08/2017 GE2017: Can you trust that surprise exit poll?https://theconversation.com/ge2017-can-you-trust-that-surprise-exit-poll-79170 2/4Party SeatsConservative 314Labour 266SNP 34Lib Dem 14Plaid 3Green 1UKIP 0Other 18Exit poll resultsGet the dataAfter catching their breath, the first question most people are asking is whether we can trust theseresults.The short answer is yes. While the official seat count may differ in either direction due to samplingerror, exit polls have provided reliable estimates in past elections. In fact, they seemed to have faredmuch better than opinion polls in recent years for at least two important reasons.Firstly, exit polling is done at selected polling stations within constituencies with sufficiently largesamples to reduce uncertainty around the estimates. Exit polls include responses from tens ofthousands of voters, while most national opinion polls have sample sizes of around 1,200.Secondly, the exit polls only include responses from actual voters. Opinion polls must rely on voteintentions and the likelihood that a particular respondent will actually cast their ballot on electionday.However, like most opinion polls, exit polls are not based on probability samples and are thus notentirely representative of the voting public. Instead, a sample of the more than 39,000 polling stations is selected to represent the total. This process of sampling introduces error, which is why theexit poll estimates may be different than the certified seat counts.How well did the pollsters do?The figure below provides a snapshot of election forecasts on June 7 2017.16/08/2017 GE2017: Can you trust that surprise exit poll?https://theconversation.com/ge2017-can-you-trust-that-surprise-exit-poll-79170 3/4UK politics Polls Polling methodology Polling Pollsters UK election 2017The mean projection based on the polls ahead of voting was 348 seats for the Conservative party, witha difference of 69 seats between the most optimistic prediction and the least optimistic one. If the exitpoll is correct, then YouGov’s model predicting 302 Tory seats appears to have performed best amongthe major forecasters. That would suggest YouGov’s sophisticated multilevel regression and post-stratification method (“Mister P”) was effective. More importantly, it appears that YouGov used abetter turnout model than other pollsters to weight the data (to correct for who would turn out to votein this election).In recent elections, pollsters have systematically underestimated vote share for the Conservative partyby roughly 5%. This time around, they seem closer to the mark, and some may have evenoverestimated the party’s final vote share.One final issue is worth noting. Even experts find it extremely difficult to predict election outcomes. Afew weeks ago (May 16 to the 26), 335 election experts provided their projections in the first Political Studies Association expert survey. Although these predictions were taken several weeks ago anddidn’t allow for recent changes in the campaign, the experts predicted a Conservative seat count of371. Regardless of the final count, one thing seems clear: a Conservative seat count of 371 willprobably be some way off.Found this article useful? A tax-deductible gift of £20/month helps deliver knowledge-based,ethical journalism.How the polls stacked up. Author provided 16/08/2017 GE2017: Can you trust that surprise exit poll?https://theconversation.com/ge2017-can-you-trust-that-surprise-exit-poll-79170 4/4Make a donation

GE2017: Can you trust that surprise exit poll?

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GE2017: Can you trust that surprise exit poll?

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