Polls apart: What on Earth was going on with the contradictory numbers?

Analysis Ian Jones

One of the things for which the 2017 election campaign will be most remembered is the surprising and often contradictory behaviour of the opinion polls. How have the polls changed during the campaign? On 19 April, the day after Theresa May called the election, the Conservatives’ poll lead over Labour averaged 17 points. On the final day of the campaign, the lead averaged seven points. No election in modern times has seen such large movement in the polls over such a short period. What caused the Tory lead to fall? A big jump in Labour’s share of the vote. The party moved up from an average of 26 per cent n 19 April to 37 per cent. The Conservatives’ share of the vote has remained roughly static - it averaged 43 per cent on 19 April. Where has Labour’s support come from? The polls suggest there was a shift in support between a number of parties. Since the campaign began, Ukip saw its average poll rating drop from 11 per cent to 4 per cent, the Lib Dems from 10 per cent to 8 per cent and the Greens from 4 per cent to 2 per cent. It is likely that both Labour and the Tories benefited from these shifts. The Conservatives’ static poll rating could also mask a lot of “churn” in support, with some voters moving from Ukip to the Tories while others move from the Tories to Labour. Why did individual polls been give the Conservatives such contrasting leads? In the last week of the campaign we saw Conservative leads ranging from a single point (from the polling company Survation) to 12 points (Comres). The difference was due to the way pollsters compile their data and produce their numbers. Some adjust their figures based on past voting behaviour. Others try to make allowances for turnout. ICM, for example, calculates its figures on an assumption that younger people will be less likely to vote than older people, and less affluent people are less likely to vote than the wealthy. What do the polls tell us about how young people might have voted? There is a huge difference in voting intention between young and old. The most recent poll to be published by Yougov, conducted between 1 and 2 June, showed support for Labour among 18 to 24-year-olds to be at 71 per cent, while the Conservatives were on 15 per cent. By contrast, people aged 65 and over divided 62 per cent for Conservatives and 19 per cent for Labour. Whether that made a difference on polling day depends a lot on turnout.