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Editor's factfile: Monday April 18
What are opinion polls and why are they currently sending such mixed messages?
Opinion polls are a snapshot of opinion at any given time but they should not be read as gospel.
Pollsters go to great lengths to point out that polls don't predict the outcome of a general election. They simply reflect the state of the public's opinion at the point where the survey is conducted.
But they are very influential and despite what politicians say in public they do look to them to assess how well, or badly, they are doing in the country. Indeed parties have historically commissioned private polling to assess their position within the nation's affections.
Polling is now as integral a part of elections as ballot papers, stubby pencils and drafty church halls. Despite that they often send very mixed messages.
One poll this weekend gave Labour a lead heading into double digits. Yet another, the YouGov poll, gave the government a single point lead.
There may be methodological differences - such as the fact that some organisations poll face-to-face while others do so on the internet or by telephone - but critics say the disparity is indicative of the inherent flaws in political opinion polling.
Timing can be important. Polls conducted by day may miss a large body of the working population - who may be more inclined to support one party rather than another. This was the case in the 1992 general election when newspaper chiefs wanted up to the minute polling data as they went to press early in the evening.
The interesting element of any poll should be the trend. If an ICM poll which last month showed Labour ahead now shows the Tories ahead then it's fair to assume something is on the move.
But if a MORI poll shows Labour ahead, while a week later a YouGov survey shows a Tory lead, don't assume necessarily that public opinion has changed. There could be a more systematic reason why different polling organisations reach different conclusions.
Remember also that opinion polls come with a big old caveat. Not only are they a transient snapshot, they are also subject to a two or three per cent margin of error.
So a result putting Labour on 40 per cent and the Conservatives on 30 per cent could mean Labour support is somewhere between 37 and 43 per cent, while the Tories are on 27 to 33 per cent. In other words, the gap between the two parties could be anywhere between four points and 16 points.
In some countries, including France, the publication of poll data is banned in the days before an election.
An election and opinion poll weary public may come to wish the same restrictions were applied here.
But they generate 'exclusive' headlines for newspapers and more publicity for polling firms.
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