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Election exit poll: Not quite 'spot on' this time, but another triumph for statistical methods!
The exit-poll design and analysis methods developed by David Firth (with political scientist John Curtice from the University of Strathclyde) were used again at this week's General Election by all of the major UK broadcasters.
At 10pm on election day the on-air seats prediction (simultaneously on BBC, ITV and Sky) based on the exit poll was: Con 316, Lab 239, SNP 58, LD 10, others 27. The actual result of the election was Con 331, Lab 232, SNP 56, LD 8, others 23.
The 2015 exit-poll prediction was thus not "spot on" as it had been in 2005 and 2010. Many commentators had warned beforehand that the 2015 election would be an especially difficult one to predict. The exit-poll prediction was startlingly different from what had been indicated by commercial pre-election voting-intention polls. (e.g., see The Observer on 10 May, After the exit poll, a tsunami raged across the political map) The exit poll strongly indicated the Conservatives as largest party, and the ultimate outcome of a small Conservative majority was clearly not ruled out. This was in stark contrast to predictions from pre-election polls, which had consistently shown Conservative and Labour neck-and-neck with neither party close to an overall majority.
There were some notable public quotes, most prominent of which came from the former Liberal Democrat leader Lord Ashdown, who was interviewed soon after 10pm on the BBC:
If this exit poll is right, I will publicly eat my hat on your programme.
(He was referring to the predicted collapse of the Liberal Democrats to just 10 parliamentary seats. In the event, it turned out even worse than that for the Liberal Democrats, who won just 8 seats. Lord Ashdown failed to keep his hat-eating promise, though!)
For more information on the methods and their performance at previous UK general elections, see Exit Polling Explained.