Explainers

The night of the general election in 2015 saw widespread shock on the faces of political campaigners, journalists and the public as the predicted hung parliament morphed before our eyes into a Conservative majority.

For months the polls had consistently shown Labour neck-and-neck with the Conservatives, leading to a slew of opinion pieces on how to navigate a hung parliament, what a minority government would mean for the UK, and whether Ed Miliband was quaking in his boots at the thought of having to work with the SNP.

But the polls were incorrect - the Conservatives stormed ahead with 331 seats to Labour’s 232 and formed a majority government. Both of these figures were over 40 seats away from their predicted seat count – a huge margin of error. Although the vote share predictions for the other parties were reasonably accurate, we cannot ignore the fact that polling failed to represent the true state of our two major political parties.

"The problem is that when the polls are wrong, they tend to be wrong in the same direction." – Nate Silver

In this post, we will dig a little deeper into the strange world of opinion polls. One of the challenges mentioned in our first article about polling was the difficulty of randomly finding a group of people whose views reflect the overall population. To use polling terminology, finding a representative sample of people to answer your questions can make the difference between a poll being spot on or wildly inaccurate.

This blog looks at two techniques that pollsters deploy to find a representative sample, random dialing and panels. It also looks at how the use of something called weighting can be applied to poll results to make them more representative.

"Not everything that counts can be counted, and not everything that can be counted counts."

Elections mean many things – campaign rallies, manifestos, a debate about whether there will be a debate, and, of course, polls.

Polls generate daily headlines in an election – even, or especially, when they contradict each other. Despite failing to accurately predict the result of either the 2015 general election or the Brexit referendum, a new poll can still be front page news. Public trust in opinion polls has plummeted, with one survey finding that 75% of adults don’t trust surveys.

So, how does polling work? Why is it so difficult to get right in the UK? Our goal is to explain enough essentials to get you through most conversations on election polling, as well as enabling you to cast a critical eye on the headlines that polls generate between now and the election.

How is it possible that an opinion poll claims to know what the whole country thinks just by asking a few hundred people?

Ask a hundred people if they like cats and around 63% will say yes, give or take a few. If you want to be more accurate, you can even say that the number will be between 53 and 73. What if you asked a thousand people? Perhaps you’ll find 530 to 730 ailurophiles this time?

"Wisdom is the daughter of experience", according to Leonardo da Vinci, and experiencing more responses to our cat question means we can actually make a wiser guess - with a sample size of one thousand people you should find that 600 to 660 of them like cats.