The piece by Anthony Wells on postal voting and opinion polls on UKPR is a good reminder that things can now get a bit complex given that perhaps one in five of all votes will be made by post.
What happens when the pollster calls or someone fills in an internet form when they have already voted? Almost all the firms adapt their voting intention question to take into account this possibility but they cannot report them separately.
As Anthony writes: “It is against the law to publish any poll based on people saying how they have already voted until the polling stations close on May 6th, and this includes people who have cast postal votes. In the European elections in 2004 Populus and the Times were investigated by the police over this for publishing voting figures for regions where there were all-postal ballots, which were hence effectively exit polls. No action was taken since it was not clear beforehand that this was against the law, but guidelines on how the law would apply to such things were subsequently drawn up. The result was that pollsters can include people who have already cast postal votes in their figures, but it is illegal to separately report figures for just those who have already cast postal votes.”
Theoretically at least you would expect polls to be that more accurate for in they will become partial exit polls.