General election: Can we trust the opinion polls?

Polls have come under fierce scrutiny after many were wide of the mark on the EU referendum and last two general elections

Can you trust the polls?

As the country braces for the unexpected pre-Christmas election, it’s an important question, especially as some voters will be seeking advice on how to cast their ballot after the recent political turmoil.

Opinion polls have come under fierce scrutiny of late, after failure by most to predict the result of the Brexit referendum, the 2015 election, Donald Trump’s victory in the US and the hung parliament in 2017.

Journalists breathlessly follow the polls and the political parties keep a close eye on how they fare – but it can sometimes feel like a lot of noise to voters.

Polls are certainly worth considering but it is important to look at the fine print and to treat any survey as a snapshot of a continually evolving picture.

Reputable pollsters such as YouGov and BMG Research – which performs polls for The Independent – are members of the British Polling Council (BPC), which has strict rules governing how surveys are conducted.

The BPC also investigates complaints from members of the public of perceived inaccuracies in polls.

When looking at a poll, always consider who has paid for it. If it is for a campaign group, there may be loaded questions or selective findings – but if it is carried out by a reputable pollster then it could still be a reasonable survey.

Sample size is important when considering a poll’s significance. As a rule of thumb, the survey should be of at least 1,000 people to get a reasonable spread of views – although doubling the sample size, does not necessarily double the quality of the research.

Pollsters use either “random” sampling, where people are chosen from the electoral register, or “quota” sampling, where they identify demographic categories such as gender or age to poll.

They may adjust their findings to ensure the sample is representative of the profile of the population, which can be compared with census data.

Veteran pollster Peter Kellner compared the sampling system to how a chef can judge the taste of a vat of soup through a single spoonful.

In a BPC briefing, he said: “Achieving representative samples is broadly akin to stirring the soup. A non-scientific survey is like an unstirred vat of soup. A chef could drink a large amount from the top of the vat, and still obtain a misleading view if some of the ingredients have sunk to the bottom.

“Just as the trick in checking soup is to stir well, rather than to drink lots, so the essence of a scientific poll is to secure a representative sample, rather than a vast one.”

Decent polls are supposed to have a 3 per cent margin of error but they can still provide a useful snapshot of general public opinion, even if the result is further out than that.

The snap election is expected to be particularly tricky due to increased volatility among voters.

Brexit has become such a dividing line that some people will undoubtedly eschew traditional party loyalties at the ballot box.

Best for Britain, the anti-Brexit campaign, was at the centre of a row over its new tactical voting tool, which advised pro-EU voters to back the Liberal Democrats in some Tory/Labour marginal constituencies.

However the campaign argued that 2017 electoral figures are out of date and their modelling showed that the electorate was more fluid than ever.

The only way to really know the way people will vote is to read the news on the morning of 13 December.

But in the meantime, read the polls with clear eyes, always ask questions – and buckle up for a bumpy ride.