If there is to be another election, those in politics, the media and wider society need to become more careful and sceptical consumers of pollsby Will Jennings / September 6, 2017 / Leave a comment
The performance of the opinion polls in the 2017 general election was, much like the experience of 2015, as they say, ‘sub-optimal’.
This was a highly unusual election in terms of polling, though. It was unusual firstly because of the size of the surge in Labour support during the campaign—which exceeded the change in voters’ intentions in all other elections since 1945. It was also unusual because—seemingly unnoticed by most—the polls actually got the Conservative vote share right for once!
This historical tendency had been for the polls to under-estimate Conservative support and over-state Labour. For once this did not happen, with Labour’s vote under-estimated by 5 percentage points on average. In fixing an old problem, the pollsters discovered a new one: cranking the methodological levers a little too hard and reducing the Labour vote share. By contrast, the UKIP vote was over-estimated quite substantially—something which has to date been given little attention, despite its potential importance for the final result.
It was the combined polling error, missing the Conservative-Labour lead, that was the cause of surprise when the exit poll was released at 10pm on Election Night. In historical perspective that error was not out of the ordinary—it had been larger in the general elections held in 1951, 1970, October 1974, 1992, 1997 and 2015. What mattered was that it critically tilted the balance of seats in parliament: Theresa May’s Conservatives failed to win a majority, undermining the entire reason for calling the election.
Putting the exit poll in context
Compared to other countries, the historical record of polling in Britain is in fact relatively…