How general election polls have changed during campaign

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One of the things for which the 2017 general 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?

Labour leader Jeremy Corbyn. Picture: Scott Heppell/PA Wire

Labour leader Jeremy Corbyn. Picture: Scott Heppell/PA Wire

On April 19, the day after Theresa May called the election, the Conservatives’ poll lead over Labour averaged 17 points. Today, the final day of the campaign, the lead is averaging seven points. No election in modern times has seen such large movement in the polls over such a short period.

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What has caused the Tory lead to fall?

Prime Minister Theresa May visits Clockwork Removal and Storage company in Edinburgh in the build up to the UK general election. Picture: Ian Georgeson

Prime Minister Theresa May visits Clockwork Removal and Storage company in Edinburgh in the build up to the UK general election. Picture: Ian Georgeson

A big jump in Labour’s share of the vote. The party has moved up from an average of 26% on April 19 to 37% today. The Conservatives’ share of the vote has remained roughly static - it averaged 43% on April 19 and is currently 44%.

Where has Labour’s support come from?

The polls suggest there have been shifts in support between a number of parties. Since the campaign began, Ukip has seen its average poll rating drop from 11% to 4%, the Lib Dems from 10% to 8% and the Greens from 4% to 2%. It is likely that both Labour and the Tories have 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.

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An average poll lead of seven points suggests the Tories are on course for a win - is that right?

The Tories finished roughly seven points ahead of Labour at the 2015 general election, so a similar lead this time could translate into a similar result - a very small Tory majority. But polls are snapshots of opinion, not forecasts, and things could look very different once the actual results come in.

Why have individual polls been giving the Conservatives such contrasting leads?

In the past week we have seen Conservative leads ranging from a single point (from the polling company Survation) to 12 points (ComRes). The difference is 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 vote?

There is a huge difference in voting intention between young and old. The most recent poll to be published by YouGov, conducted between June 1 and 2, shows support for Labour among 18 to 24-year-olds to be at 71%, while the Conservatives are on 15%. By contrast, people aged 65 and over divide 62% for Conservatives and 19% for Labour. Whether this makes any difference on polling day will depend a lot on turnout. At the 2015 general election just 43% of 18 to 24-year-olds were estimated to have voted, compared with 78% of people aged 65 and over.

What needs to happen for there to be a hung parliament?

If the Conservatives finish around three points ahead of Labour - say, 40% to Labour’s 37% - that could be enough, if reflected across the country on polling day, for the Tories to lose their majority in parliament. They would still be the largest party, however, and therefore in the best position (as in 2010) to try to form a coalition. The Tories could still be the largest party in a hung parliament even if Labour finishes ahead of them in the share of the vote. For example, if Labour finished on 40% and the Tories on 37%, this would translate into a hung parliament with the Tories the largest party but around 35 seats short of a majority. Labour needs to be at least six percentage points ahead of the Conservatives to be sure of being the largest party in a hung parliament.

And what about Labour winning an overall majority?

To be sure of that happening, Labour would need to finish at least 12 percentage points ahead of the Conservatives.