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.

The broadcasters’ exit poll was, once again, accurate. (Image: BBC)

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…