Sir Robert Worcester is a Visiting Professor of Government at LSE and an Honorary Fellow. He founded MORI in 1969.

The blogs are at it again, even letter writers to newspapers.  A headline in the Standard the other day caught my eye. “Election polling is too uncertain.”  It would, wouldn’t it.

From the writer’s first sentence, “With regard to the variable picture on support for the parties in different opinion polls…”, with respect, undermines the rest of his argument.  The media’s fixation on the lead, rather than the share of support for each party is the principle cause of his confusion. With the exception of the two ‘new boys on the block’, Angus Reid and Opinium, since the beginning of the year there have been some 59 polls (counting the daily onslaught from YouGov as one every two days).  The Conservative share of voting intention has been  38 per cent plus or minus the usual three percent margin of error in all 59, for a 100 per cent ‘Stability Index” rating.  See embedded spreadsheet below (you can zoom in or show the table in full screen using the buttons below the embedded spreadsheet):

General Election Voting Trends January – 5 April 2010

The figures for Labour are somewhat less than perfectly within the +/- 3 per cent: only 97 per cent consistent.  The figures for the Liberal Democrats is somewhat better: 98 per cent.

The April Indices are even better so far: four out of four at 100 per cent, Tories 38 per cent, Labour 30 per cent, LibDems 20 per cent and Others 12 per cent.

The conclusion of the letter writer, a distinguished academic whose name I shall not mention here, is that “Our (sic) polls, built for an earlier period, are now hopelessly out of date”. He asks more of polls than the laws of statistics and the vagaries of the public interviewed over several days and at different times, can ever deliver, as it is an impossible task.  They are what they are, the best approximation of the state of public opinion at the time they were taken, better than anyone’s guess, as they are done by honest people, doing their best with the tools of their trade, the public, the science of sampling, and the art of asking questions.

You can think of it this way.  If we were to send out our interviewers to a hundred constituencies, carefully selected to give a random representation of all the British constituencies, and ask each interviewer to stand at the highest point in the constituency and at precisely 12 noon enter the temperature from an accurately tested thermometer into their computer and send it into the central computer to be aggregated, you would have the average temperature of the country to within a fraction of a degree.

Then if the next day you asked them to exactly replicate this and send the next day’s result in, would you expect the same reading?  No, of course not, everyone will understand that. So why is it that anyone fails to understand that such a mercurial thing as public opinion cannot be expected to be any more accurate than the temperature?

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