The level of support for candidates in a Parliamentary by-election the weekend before polling day is only a modest guide to the likely final result. That’s the lesson the Liberal Democrats repeatedly demonstrated during the party’s by-election winning heyday.
Whether tracked through public data such as opinion polls or private data such as canvass returns the picture was often the same: a huge swing in the last few days.
Which is why talk now about the opinion polls in Heywood and Middleton were “wrong” to show a large Labour lead, when in fact Labour came so close to losing it there was a bundle check, is itself wrong.
The polls were, on the scale on which large by-election movements of opinion take place, conducted a long way out from polling day. The election was on Thursday 10 October but the opinion poll fieldwork concluded on 4 October (Lord Ashcroft) and 30 September (Survation).
In calmer by-election times that works. In the past that would have often failed – so the first reaction to the polls in Heywood and Middleton shouldn’t be to say they were wrong, but that they were too early.