Election Matters: Why do opinion polls matter - and can we trust them?

ITV News Election Analyst Professor Jane Green explains how polls work, whether we can trust them, and how they do much more than create headlines

Take a second to imagine a world where opinion polls didn't exist.

Without them we'd have no real idea about what everyone else in the country thinks before and during an election - nor how popular any one party is.

Anyone, and any politician, could say what they liked about how popular they are and what concerns voters.

But they'd have to base these claims on pretty sketchy evidence.

Why do we need opinion polls?

Polls get a lot of attention in politics. For example, right now, one of the top issues is the NHS.

These results can tell politicians which problems are cutting through; and if they ignore those issues, its typically at their peril as they will look to the public as being really out of touch.

Polls also tell the media which parties need to be represented in their coverage.

Can we trust polls?

Generally speaking, opinion polls tend to be pretty good at what they do.

They also become more predictive of the election the closer we get to it, as we make up our minds about who to vote for.

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How do polls work?

Pollsters commit to transparency and codes of conduct, and take a sample - usually around 1,000 people - while ensuring their sample is as good a match as possible to the UK demographic as a whole.

Interviews for polls are usually conducted online - possibly on the phone.

The key to a good poll is the quality of the sample or, in other words, how well it matches the make-up of the UK, as opposed to the number of people interviewed. 

Think of polls you see on social media. While thousands of votes may be cast, they could all be from one niche group who all think the same.

How do we make sense of polls?

The key thing with looking at opinion polls - and there are loads coming out at the moment - is to wait to see the signal that emerges rather than the inevitable noise.

This means waiting to see the pattern that emerges instead of focusing on one individual poll.

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Do polls help voters?

Polls can influence how some people vote or whether they vote at all. For example, the result could look so obvious that some people don't bother turning up on election day.

But there are reasons to be careful as polls do not always get things exactly right.

Strong support in polls does not always mean a party gets the largest number of MPs.

Smaller parties have to really concentrate that support in constituencies just to get one MP, whereas large parties need their vote to be widely spread out.

Can the local picture influence votes?

Many people take the local race into account: who has the best chance of winning in their constituency.

If you want to factor this into your decision, don't look at national polls, but instead at recent results where you live.

So, polling is useful if you know how to use polls. But at the end of the day, what really matters is the election result itself.

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