J Curtice and D Firth
Exit Polling in a cold climate: The BBC/ITV experience in Britain in 2005
Final version published online 3 Sept 2007. Accepted in July 2007 for reading at RSS Ordinary Meeting on 17 Oct 2007, and for subsequent publication (with discussion) in J. Roy. Statist. Soc. A. (link)
Abstract: Conducting an exit poll to forecast the outcome of a national election in terms of both votes and seats is particularly difficult in Britain. No official information is available on how individual polling stations voted in the past; use of single member plurality means that there is no consistent relationship between votes and seats; electors can choose to vote by post; and most of those who vote in person do so late in the day. In addition, around one in every six intended exitpoll respondents refuses to participate. Methods developed to overcome these problems, and their use in the successful 2005 BBC/ITV exit poll, are described and evaluated. The methodology included: a panel design to allow the estimation of electoral change at local level; coherent multiple regression modelling of multiparty electoral change to capture systematic patterns of variation; probabilistic prediction of constituency winners in order to account for uncertainty in projected constituency level vote shares; collection of information about the voting intentions of postal voters before polling day; and access to interviewer guesses on the voting behaviour of refusals. The coverage and accuracy of the exit poll data are critically examined, the impact of key aspects of the statistical modelling of the data is assessed, and some general lessons are drawn for the design and analysis of electoral exit polls.
Keywords: general election, exit poll, forecasting, postal voting, probability calibration, Steed swing, swingometer, ternary diagram