Lies and statistics? The pollsters who make the numbers add up

Predicting who'll win an election is an inexact business, not least because voters don't always tell the truth. Rob Sharp meets the pollsters who make the numbers add up

Do you remember where you were on 10 April 1992? Robert Worcester, founder of Ipsos MORI, certainly does. It was the day after John Major led the Conservatives to their fourth consecutive election victory.

None of the pollsters had seen it coming. The day before the election their data had suggested Labour would narrowly beat the Tories by one percentage point, the equivalent of eight Commons seats, resulting in a hung parliament. Instead, Major's party took an eight-percentage-point lead, or 65 seats, resulting in an overall majority of 21. It was one of the biggest upsets in polling history.

"It was chaos," says Worcester. "There were five principal polling organisations and three refused to talk to the media. I don't believe in maintaining silence, so I was one of the few people interviewed on TV and ended up bearing the brunt of the blame ... I said we didn't know what we'd done wrong, but that we wouldn't leave a stone unturned until we found out."

The Market Research Society, representing the pollsters, looked into what went wrong in 1992, and blamed inaccurate data about the socio-economic breakdown of the population.

As the number-crunchers gear up for another general election, are they now more confident of their data? This month polling company ComRes reported that the Tories had slumped to 38 per cent of the vote, just seven percentage points ahead of Labour, on 31 per cent, with the Liberal Democrats at 19 per cent. Like the predicted result for 1992, this would result in a hung parliament.

So how have ComRes arrived at this latest prediction? The raw data is these days is gathered over the phone (pollsters claim people are more likely to tell the truth than in face-to-face interviews). At the ComRes offices on Millbank, around 30 employees sit behind rows of computer monitors, some busy on the phone, others tapping numbers into databases. Most polling organisations will speak to a sample of 1,000 people per opinion poll. The bigger the sample, the better chance of accuracy. With a sample of 1,000 people, there is a 95 per cent chance of accuracy to within the margin of error, in this case plus or minus three per cent.

Most polling organisations select these 1,000 interviewees randomly from the records of telephone numbers. Of the major polling organisations – ComRes, ICM, Ipsos MORI, Populus and YouGov – only YouGov relies instead on samples taken from its own pool of 300,000 'panel members' – and conducts its polls online. But any sample of 1,000 people may be unrepresentative because it features, say, too many habitual Labour- voters, or too many people from lower-income groups, or too many from certain regions. So the pollsters put the responses they collect through a process of "weighting".

For example, if a sample of 1,000 people is made up of 550 men and 450 women, it doesn't accurately reflect the British population (51 per cent female). To compensate, the answers of female respondents will be given slightly more "weight" (in this case they will each count as 1.133 people). The same procedure is carried out to adjust for age, social class and region. Pollsters will also take into account how people voted in the previous election, and a person's statistical likelihood of actually casting his or her vote.

Pollsters then convert their results into seats in the House of Commons. In carrying out this conversion, online calculators such as that at electoral- calculus.co.uk can also factor in the effect of marginal constituencies, where a seat is most likely to change hands, as well as the effects of tactical voting. Back in 1992, the pollsters had inaccurate information about socio-economic groups partly because the latest census results were 11 years out-of-date; we also now know that many people who in the event voted Conservative lied to the pollsters about their voting intentions. These days pollsters argue that weighting according to past vote gets around that bias.

"Historically, over the longer term, pollsters have had a mixed record," admits ComRes founder Andrew Hawkins. "But in 2005, within 48 hours of the election every eve-of-election poll was [accurate to] within three per cent. This is strikingly accurate given that the journey from poll question to results is not a straightforward one. It's not like using a dipstick on a car engine. It's more of a modelling process." Every pollster, he says, should "show a willingness to change their methods over time."

Do we have any choice but to listen to the pollsters? Not really, says John Rentoul, The Independent on Sunday's chief political commentator: "We don't have any alternative way of finding out what the electorate is thinking, apart from focus groups, but they don't deliver the numbers." But some pollsters will never lay the ghosts of 1992 to rest.

"I'm not confident that we can't have another 1992 election, in the same way I can't be confident that journalists aren't going to make any more mistakes," says Robert Worcester. "You can't be 100 per cent sure that lightning isn't going to strike twice. It's statistics, and I spend a heap of my life explaining this. It's the marriage of asking questions with the science of sampling. I'd always say that it's 95 per cent expertise and five per cent luck. And if you're not lucky, then you're in the wrong business."

Start your day with The Independent, sign up for daily news emails
Have you tried new the Independent Digital Edition apps?
ebooks
ebooksA special investigation by Andy McSmith
2015 General Election
May2015

Poll of Polls

Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: IT Manager

£30000 - £40000 per annum: Recruitment Genius: This distributor of electronics...

Recruitment Genius: Personal Assistant

£18000 per annum: Recruitment Genius: Our client has a number of different bus...

Recruitment Genius: Frontend Developer / Designer

£20000 - £30000 per annum: Recruitment Genius: You will be working on a range ...

Ashdown Group: Software Developer - C#, .Net, SQL - Cambridgeshire

£30000 - £40000 per annum + benefits: Ashdown Group: A highly successful and g...

Day In a Page

HIV pill: Scientists hail discovery of 'game-changer' that cuts the risk of infection among gay men by 86%

Scientists hail daily pill that protects against HIV infection

Breakthrough in battle against global scourge – but will the NHS pay for it?
How we must adjust our lifestyles to nature: Welcome to the 'Anthropocene', the human epoch

Time to play God

Welcome to the 'Anthropocene', the human epoch where we may need to redefine nature itself
MacGyver returns, but with a difference: Handyman hero of classic 1980s TV series to be recast as a woman

MacGyver returns, but with a difference

Handyman hero of classic 1980s TV series to be recast as a woman
Tunnel renaissance: Why cities are hiding roads down in the ground

Tunnel renaissance

Why cities are hiding roads underground
'Backstreet Boys - Show 'Em What You're Made Of': An affectionate look at five middle-aged men

Boys to men

The Backstreet Boys might be middle-aged, married and have dodgy knees, but a heartfelt documentary reveals they’re not going gently into pop’s good night
Crufts 2015: Should foreign dogs be allowed to compete?

Crufts 2015

Should foreign dogs be allowed to compete?
10 best projectors

How to make your home cinema more cinematic: 10 best projectors

Want to recreate the big-screen experience in your sitting room? IndyBest sizes up gadgets to form your film-watching
Manchester City 1 Barcelona 2 player ratings: Luis Suarez? Lionel Messi? Joe Hart? Who was the star man?

Manchester City vs Barcelona player ratings

Luis Suarez? Lionel Messi? Joe Hart? Who was the star man at the Etihad?
Arsenal vs Monaco: Monaco - the making of Gunners' manager Arsene Wenger

Monaco: the making of Wenger

Jack Pitt-Brooke speaks to former players and learns the Frenchman’s man-management has always been one of his best skills
Cricket World Cup 2015: Chris Gayle - the West Indies' enigma lives up to his reputation

Chris Gayle: The West Indies' enigma

Some said the game's eternal rebel was washed up. As ever, he proved he writes the scripts by producing a blistering World Cup innings
In Ukraine a dark world of hybrid warfare and murky loyalties prevails

In Ukraine a dark world of hybrid warfare

This war in the shadows has been going on since the fall of Mr Yanukovych
'Birdman' and 'Bullets Over Broadway': Homage or plagiarism?

Homage or plagiarism?

'Birdman' shares much DNA with Woody Allen's 'Bullets Over Broadway'
Broadchurch ends as damp squib not even David Tennant can revive

A damp squib not even David Tennant can revive

Broadchurch, Series 2 finale, review
A Koi carp breeding pond, wall-mounted iPads and a bathroom with a 'wellness' shower: inside the mansion of Germany's 'Bishop of Bling'

Inside the mansion of Germany's 'Bishop of Bling'

A Koi carp breeding pond, wall-mounted iPads and a bathroom with a 'wellness' shower