How reliable are exit polls at telling us who will win the General Election?

The exit poll has predicted a big Tory majority in what has been dubbed  ‘the most important election of a generation.’

Seats have not been counted, but pollsters predict the Tories will win 368 seats, Labour 191 seats, the Lib Dems 13 seats, the Greens one seat and SNP 55.

If correct, it indicates the Conservatives will have a thumping majority of 86 seats.

Polls yesterday suggested the result was at a ‘knife edge’, with both a Tory majority and a hung parliament likely.

The exit poll does not guarantee the outcome but is usually taken as an accurate indicator of how the results will go.

So what is the exit poll and just how accurate is it ?

What is the exit poll?

The exit poll is a survey of thousands of voters just after they have cast their ballot.

It covers 144 constituencies in England, Scotland and Wales but not Northern Ireland, where a different set of parties dominate politics.

Exit pollsters base themselves at selected polling stations in different constituencies, which are chosen to be demographically representative of the country and weighted slightly in favour of marginal areas.

Voters are asked to fill in a replica ballot paper, the results of which are analysed by experts at secret location in London.

How accurate is the exit poll?

In recent years it has been a very good predictor of the election result.

In 2017, the exit poll predicted the Tories would end up with 314 seats, just four short of the 318 the party actually won.

Labour was forecast to win 266 (they finished on 262), the Lib Dems 14 (12), the SNP 34 (35), Plaid Cymru 3 (4), Ukip 0 (0), the Greens 1 (1) and others 18 (18).

The rough rule of thumb is an exit poll that comes within 15 seats of the final outcome is considered accurate.

Past exit polls

The first British exit poll, in 1974, predicted a Labour majority of 132, but the actual majority was three.

Another misfire happened in 1992, when two separate exit poll  for the BBC and ITN, both predicted a hung Parliament.

Instead, John Major’s Conservative government held its position, albeit with a significantly reduced majority.

Over the decades, and under the supervision of political scientist Professor Sir John Curtice, its predictions improved.

However there was a recent blip in the 2015 election, which predicted a hung parliament rather than a Conservative majority.

How quickly will we know if the exit poll was right?

It may take some time for the full picture to emerge but if there’s a strong trend, the first seats to declare, traditionally in the north-east of England, will confirm the exit poll’s prediction – or not.

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