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Expert comment: The accuracy of exit polls

Professor David Firth, from Warwick's Department of Statistics is the inventor of the statistical methodology of the exit poll used by the main UK broadcasters on election night. He comments on its extraordinary accuracy in previous years but states it may not be as accurate every time.

"The innovative use of statistical modelling is what transformed exit polling at UK general elections, from a rather hit-and-hope exercise (in the 1990s and earlier) to an activity whose on-the-night predictions are now much more likely to be fairly accurate. Still, any exact prediction of seats won by the largest party, such as seen in both 2005 and 2010, owes as much to luck as it does to sound statistical thinking. There is nothing in the new methods that guarantees such freakish accuracy! Indeed, even getting a prediction error as small as four seats — as seen at the most recent General Election in 2017 — has to be regarded as extraordinarily accurate.

"More typically the exit poll ought to be expected to predict with an error in roughly the five to 15 seats range (for the main parties). Sometimes the error will be smaller than that (as seen in 2005, 2010 and 2017); and occasionally it might be larger.

"But the main point here is this: While better methodology has radically improved the chances of an accurate prediction from the exit poll at a UK General Election, the super-accurate predictions seen in 2005, 2010 and even 2017 were unwarrantably accurate. Such an astounding level of accuracy is not guaranteed by the statistical methods used, and it definitely should not be expected every time!"

For more information on Professor Firth's exit poll work, visit his Exit polling explained page. 

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