The Long Odds of the Opinion Polls

May 7, 2010 Updated: October 1, 2015

Political opinion polls are not as reliable as weather-forecasting but used almost as often. (Bruno Vincent/Getty)
Political opinion polls are not as reliable as weather-forecasting but used almost as often. (Bruno Vincent/Getty)
LONDON—Hardly a week has passed for the last couple of months without an opinion poll soothsaying the outcome of the election. And yet, despite their close following in the media, opinion polls remain an unreliable art, less reliable than the fortune-telling power of the bookies' odds.
The past elections are littered with the failings of opinion polls. And even when they have picked the winner, opinion polls have still been way off the mark in predicting the level of support.

In the 1997 general election, although the change of power from John Major's Conservatives to Tony Blair's New Labour was anticipated, the degree of the swing in voting was overestimated.
Final numbers by Gallup, Harris, MORI, and NOP were out by 6 to 8 per cent. Another big pollster, ICM, came quite close to the 13 per cent difference between the parties. Close enough for Nick Sparrow of ICM to confidently publish an appreciation of this failure of his industry.
But by identifying the winning party, the opinion polls in 1997 largely obscured their numerical failing, thus avoiding the derision of the 1992 opinion poll predictions.
In the 1992 General Election, final pollster figures were out on average by over 8 percentage points: the Conservative vote was forecast as 4 per cent less than it turned out to be and Labour support was overestimated by a similar amount. The Tory victory was not forecast.
Since that time, says Dominic Lawson, the polling organisations have sought to counter what is perceived by some to be an endemic under representation of the Tory vote in polling samples.
Writing in The Independent Mr Lawson said: "[A]ll the polling organisations have followed ICM by introducing various 'adjustments' which seek to make allowances for the fact that, for one reason or another, many of those who ultimately vote Conservative disguise this fact from people who ask them about their intentions."