HOW DO YOU MEASURE the accuracy of an opinion poll? Obviously, you compare its findings to the result of the election it was asking about. The closer the numbers to each party’s actual vote share, the more accurate the poll.
This is all very well for surveys carried out a few days before an election, when voters’ minds are made up and there is little time for events to intervene. But what about a poll conducted months or even years before anyone casts a real vote: how can we judge whether it is “accurate”?
Most pollsters ask people which party they would choose if an election were held tomorrow. Let’s say the survey takes place on a Wednesday, with the supposed election on the Thursday. By the usual standard, we should – on the Friday – take Wednesday’s survey and line it up against the outcome of the election that didn’t happen, and… you see the problem?
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