Confounding the commentators: how the 2010 exit poll got it (more or less) right

An exit poll conducted on behalf of the three main UK broadcasting organizations for the 2010 general election predicted that the Liberal Democrats would win fewer seats than in 2005, a suggestion that was met with widespread disbelief amongst commentators. Not only did this forecast prove correct, but the poll’s prediction that the Conservatives would be the largest party in a hung parliament with 307 seats also proved to be spot on. This paper describes and evaluates the methodology of the poll. The key features were a focus on changes in party vote shares facilitated by conducting the poll in the same places as at the previous election, modelling of the variation in those estimated changes, and a probabilistic approach to identifying the likely outcome in seats. All three features contributed to the accuracy of the 2010 poll. The data provided a highly accurate estimate of the change in the Liberal Democrat vote. Meanwhile, a slight overestimation of the swing from Labour to the Conservatives was corrected by the modelling of changes in vote shares and the use of a probabilistic approach to seat prediction.