In the run-up to the 2005 general election, there was some excitement as a rolling poll commissioned by ITV from Populus showed the Conservatives slipping to a result even worse than 1997. However, there was a sharp (5 point) recovery in the final pre-election day poll from Populus, which was a normal non-rolling poll.
Picking over the polling after the election, the explanation looked to be one aberrant sample on one day but which, due to the workings of a rolling poll, ‘infected’ the poll results over a longer period of time. It was only with the final pre-election poll, based on a wholly new sample, that this was cleansed from the figures.
That was a reminder of the perils of rolling polls – one unluckily atypical sample hangs around.
Roll the clock forward then to this week and we’ve had two consecutive days of rolling poll results from ComRes which are significantly out of line with all the other polling being conducted (placing the Conservatives in a clear lead). But once again those figures look to be down to just one day’s sample which has hung around (a point Anthony Wells has also made here).
And the commissioner of the rolling poll this time? The Independent in partnership with … ITV once again.
The wise follower of polls, of course, looks at them all in order to get the proper context.