Something I neglected to mention in my last post, is that polls can actually be designed to try to maximise the number of undecideds.
My view is that non-response is probably the most important source of error for political polls. Part of the problem is that the average person is not obsessed with politics, and they are harder to survey for this reason (because they are less inclined to take part in a poll). By targeting as high a response rate/as low a refusal rate as possible, polls are trying to maximise coverage of non-politically-obsessed people.
So if you follow this through…
- Non-politically-obsessed people are more likely to be undecided (they are more likely to say ‘don’t know’ at the party vote question).
- Poll response rates can improve a wee bit in an election year, so the proportion of undecideds may go up a bit (this is a good thing because it’s a sign a poll is getting to those who are less interested in politics)
- The undecideds may actually then decrease a bit the closer you get to the election (because some of these people start deciding).
- So the change in undecideds may have nothing at all to do with people party-switching.
In reality – a whole bunch of things will be going on, including party-switching and improved response rates.
One last thing to mention – the different polls use different definitions of ‘undecided’ so it’s not easy to compare the level of undecideds across polls, and it’s not appropriate to use this as a way to decide on the quality of a poll.