Everyone who believes the polls failed — individually and collectively — at the last election has a theory about why. Perhaps the pollsters had changed the way they found respondents and interviewed them? (Yet every mode — face-to-face interviewing, computer-assisted telephone interviewing via landlines and mobiles, robopolling, and interviewing online — produced more or less the same misleading result.) Perhaps the pollsters weighted their data inadequately? Did they over-sample the better-educated and under-sample people with little interest in politics? Perhaps, lemming-like, they all charged off in the same direction, following one or two wonky polls over the cliff? The list goes on…
But the theory that has got most traction in the post-election polling is one that has teased poll-watchers for longer than almost any of these, and has done so since the advent of pre-election polling in Australia in the 1940s. This is the theory that large discrepancies between what polls “predict” and what voters do can be explained by the existence of a large number of late deciders — voters who don’t really make up their minds until sometime after the last of the opinion polls are taken.
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