An old newspaper photograph haunts the dreams of every US pollster. A grinning Harry Truman, having won the 1948 presidential election despite every prediction, is holding up a copy of the Chicago Tribune. It reads: “Dewey defeats Trumanâ€.
Could it happen again? Every pollster is predicting a victory for Barack Obama. Might a grinning John McCain be pictured on Wednesday triumphantly holding a pile of incorrect polling data?
There are two things that say that he might.
The first is that American pollsters have not yet experienced what happened here in 1992 – when the polls pointed to a Labour victory but John Major won. The conventional wisdom is that 1992 was great for the Tories but terrible for the pollsters. In the long run, the opposite turned out to be true. Victory in 1992 turned to ashes for the Conservatives, whereas the pollsters used the debacle to get themselves sorted out.
Now British polls are properly and carefully weighted, taking account of what is known as the spiral of silence – the tendency of voters for the less fashionable party to keep their intentions to themselves. British pollsters weight their results to allow for these shy voters. US pollsters do not.
It isn’t unreasonable to believe that there could be a Republican spiral of silence. And that US pollsters are all missing it.
There is some evidence of mistakes among US pollsters. Every poll has a margin of error, to take into account the fact that a limited sample has been consulted. But the website fivethirtyeight.com has shown that during the primaries there was on average a 2.3 per cent pollster-introduced error, caused by poor methodology. This is not the case in Britain.
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linkI have to say that over here YouGov predicted the outcome to within one percentage point for the London Mayoral and local elections this year.
