Michael Williams: Readers' editor

Here we go again. The sceptic's guide to political polls

They're back. Mistrusted by many readers and ranked somewhere between estate agents and snake-oil salesmen, it was the opinion pollsters rather than the voters who saw off Gordon Brown's ambitions for an early election. It was his final scrutiny of Labour's detailed poll findings that caused the Prime minister to "bottle it", in David Cameron's words. But was he wise to trust the pollsters? Here's the sceptical readers' editor's guide to the people who will shape the next two years in politics.

1.Is there a difference between opinion polls and market research? Definitely, say the pollsters. Each has a different purpose. But with a former PR man at the helm of the Tory party, you may wonder.

2. Can a sample of 1,000 people really be representative of the country as a whole? Amazingly, yes. The maths show that sample of 1,000 people randomly generated by telephone will be accurate 19 times out of 20 to within 3 per cent. But never forget that one in 20 – and the power of Sod's Law.

3. Don't people lie to interviewers? No, say the pollsters. There's no incentive since the whole thing is anonymous. But how often have you lied to yourself when there's no incentive?

4. We hear a lot about internet polling that seems to provide "instant" results. Can such polls be trusted? They may be more accurate than conventional polling, since they tend to use "quotas", where people have provided information about themselves and so are very representative of the population.

5. Aren't polls used to set the agenda? The pollsters say not – it's those scheming journalists who choose questions to get answers they want to hear. But opinion pollsters also like to make the headlines and are often inclined to go along with what papers want?

6. Have the pollsters improved their act since 1992, when they miscalled the general election. Not entirely. In the past four general elections, the Labour vote has been overstated by about 5 per cent. This is blamed on the different behaviour of Tory and Labour voters. Tories tend to turn out in safe seats, Labourites don't. Labour voters are more likely to be deterred by the weather on a cold November morning.

7. Is there anything else in the small print I need to know? Yes – never forget the handy backside-covering mantra quoted by pollsters: "Opinion polls set out to provide a snapshot of public opinion at a given point in time and are not necessarily and automatically a predictor of future attitudes and actions."

Gordon Brown will be desperately hoping he was right to trust the polls rather than his instincts. He may well also hope that these words won't be inscribed on his political obituary.

Have your say online:

You can contact the readers' editor direct ( readerseditor@independent.co.uk), but you can also have your say on this or any other issue visit www.independent.co.uk/IoSblogs. The readers' editor reads and notes all suggestions and comments in the blogs. This column, which deals with many of the issues you raise online, is also published on the IoS blogs site.