Exit Poll Performance, 2005–2019

How has the broadcasters’ exit poll performed in the last five elections?

In the television coverage, the first major moment of the night is the seat estimates from the broadcaster’s exit poll.

The article considers the accuracy of the exit poll in the last five elections, and how uncertainty might be shown.

Performance

In the 2019 General Election, the exit poll was conducted by Ipsos MORI, and paid for by the BBC, ITV and Sky News. The fieldworkers try to go back to the same polling stations as they did in the last election. People coming out of the polling station are asked to fill out a duplicate ballot paper, replicating the vote they just made. 144 polling stations are currently part of this panel, and Ipsos MORI received 19,607 duplicate ballots on 12th December.

Analysed by a team of psephologists and statisticians, the change from the previous exit poll to the latest one is used to build a model for change between elections. This is known as the Curtice-Firth methodology of exit polling — used to overcome the major difficulty in the lack of counts by polling stations.

This exercise is all done under immense time pressure, as the exit poll results are broadcast at 10pm.

Here are the results of the 2019 General Election exit poll (compared to the actual result¹):

Conservative: 368 (365)
Labour: 191 (203)
Scottish National Party: 55 (48)
Liberal Democrats: 13 (11)

The exit poll was, once again, accurate. (Image: Ipsos MORI)

This exit poll was again accurate, with an error of three seats for the Conservatives and 12 seats for Labour.

We can look at errors in the past five elections for the Conservatives, Labour and the Liberal Democrats (with 2001 also shown):

The largest error for a single party was 15 seats. (Image: R Pubs/ggplot2)

Whilst the 2015 General Election is sometimes highlighted as the broadcasters’ exit poll failing to make the right ‘call’, the error on the Conservative seat estimate was just 15. This is the largest error for a…