Undecided voters and political polls

I’ve had some interesting posts forwarded to me over the past few weeks about polls, and how they exclude undecided voters from the party support result.

Posts at Sub-Zero Politics and The Political Scientist illustrate that poll results for party support can look quite different if undecided and/or unlikely voters are included in the base.

This is not a critique of their analyses or conclusions. I found these posts interesting, and The Political Scientist’s post inspired me to look at my own data in a different way (and that’s always a good thing). I simply want to add a few points about polling and undecided voters:

  • A poll is commissioned to estimate party support if an election was held at the same time as the poll. Given that this is the purpose, it doesn’t make sense to include those unlikely to vote in the results for party support. Also it’s not possible to include the undecideds in that result because they are, well, undecided.
  • Yes, the results would look very different if unlikely voters and undecided voters were included. But those results would look nothing at all like the result of an election held at the time of the poll, and they would be misleading in this regard. It would not be possible to translate the result into parliamentary seats – which can help to show how close an election might be under MMP.
  • Undecided voters are important. As far as I know, most polls probe undecided voters to try to get an idea of their preference. This may not make a big difference to a poll result quite far from an election – but I think it’s very important during the week prior to an election. During that week, some of the undecided voters will be paying closer attention to politics and will be starting to lean one way or another.
  • Having made the above point, it’s important to keep and mind that a large proportion of undecided voters won’t vote in an election. Based on my own analysis, about a quarter of undecided voters openly state that they don’t plan to vote. I think the true proportion would be higher than this.
  • All poll reports should state the percentage of undecided voters. It has come to my attention that these results can be hard to find. They shouldn’t be.
  • Here’s the biggie – a poll should not be expected to perfectly predict the result of the General Election. The pollsters will do their best to measure party support at the time they are polling – but they do not poll on Election Day, they do not ask ‘who will you vote for?’, they cannot predict what undecided voters will do (or whether they will vote), and there are many other factors outside their control.
  • Factors outside their control include the weather, and what politicians and political commentators do and say leading up to the election. Let’s take the 2011 election as an example. Most poll data were collected 5-7 days out from the election. In the interim, there were reports of the Prime Minister telling John Banks that NZ First supporters weren’t going to be around for much longer! It was no surprise to me that most polls tended to over-estimate National and underestimate NZ First.*

Update: *I’m not suggesting this is the sole factor behind this pattern of results.


2 thoughts on “Undecided voters and political polls

  1. I’m interested in everything about this analysis, please keep the thoughts flowing. I know quite a few undecideds and have come to think that it’s quite a lot more of a legitimate position than I used to. I think that feeling might be at large too, that because they don’t vote, they don’t count, should be ignored. But as quite a sizeable chunk of the population now, it seems that there’s a lot of opportunity being lost.

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