"Most people involved in politics know about sampling error, but, as last night, they often forget its implications."
There were four opinion polls last night, all showing different sized-Conservative leads, ranging from 3 to 10 points. Cue confusion, and lots of talk about the polls being ‘all over the place’.
In fact, it’s dead easy to explain, and the polls are actually behaving remarkably stably.
Each opinion poll has a sampling error, an acceptance that sampling 1000 or so people is not going to produce an absolute accurate measure of the views of 40 or so million.
That sampling error is usually reported along with the poll – at least by the more responsible news outlets – and it is (usually) +/- three percentage points.
And every poll last night showed the Conservatives on 37, plus or minus 3. (Actually, every poll last night showed them on 37, plus or minus 2). And every poll last night showed Labour on 30, plus or minus 3.
In fact, with one exception right at the beginning of the campaign, every poll during the campaign has shown the same: the Conservatives on 37, plus or minus 3, and Labour on 30, plus or minus 3.
The veteran pollster Bob Worcester has one key maxim: focus on the share, not the lead, and he’s spot on.
Most people involved in politics know about sampling error (anyone still puzzled, there’s a good simple summary here), but, as last night, they often forget its implications.
Professor Philip Cowley