The Curtice-Firth Methodology of Exit Polls
How do exit polls work?
At 10pm on election day, the polls close and the broadcasters reveal what the exit poll has predicted, estimating how many seats each party will take.
This moment of television is made possible by an innovative statistical method. It also relies on the hard work of polling companies, academics and journalists against the ceaseless march of time.
A model of change
Pre-election polls ask how people intend to vote. The exit poll asks people how they have actually voted, as they leave the polling station. People are asked to fill in a duplicate ballot paper, and place it inside a box.
This survey is then used to estimate seats. In 2017, the broadcasters’ exit poll was commissioned by the BBC, Sky and ITV, and conducted by GfK and Ipsos MORI. In 2019, following a partial purchase of GfK, Ipsos MORI will conduct the exit poll.
Since 2001, the exit poll followed a new methodology, and has a remarkable record of accuracy. This article explains what this method is, and its key ingredients. The method is named after Prof Sir John Curtice and Prof David Firth.
The last election results: The starting point for this method is the last election results. In each constituency, we know (with very little error) how many people have voted for each party. This is our essential resource of voter behaviour.
The previous exit poll: The reason that exit polling is extraordinarily difficult in the United Kingdom is that there is no mandate to count votes by polling stations.
The Curtice-Firth methodology crucially measures change between exit polls. Since (mostly) the same polling stations are used, biases from the choice of polling stations, differential non-response and postal votes are largely cancelled out.
The latest exit poll: The exit poll will use an experienced market and social research company, Ipsos MORI, to conduct the duplicate ballot. The exit poll panel (which polling stations are visited) is carefully planned. High-quality data underpins the entire exercise. Since the polling day occurs during a week, votes are often cast during the early evening.
