What election polls do not tell us

"...in spite of the useful information that election polls offer, they are lacking in two important respects..."

We can expect a large number of election polls to be published in the next four weeks. Not all polling agencies arrive at their numbers in exactly the same way, and they need to be read with care. And in spite of the useful information that election polls offer, they are lacking in two important respects, both of which relate to the difference between ‘hard’ choices and ‘soft’ ones.

First, consider people who respond to opinion pollsters by saying that they do not (yet) know. These respondents are generally deleted from the calculation of the parties’ projected vote shares, which implies that, were they to make up their minds, they would not disproportionally go to any one party. But consider the group of people who hesitate between the Conservatives and UKIP, and who correctly state that they do not yet know for sure which party to support. When they make up their minds, they will not support Labour, or the Liberal Democrats, or the Greens. Therefore, omitting these ‘don’t know’ responses from the results will (in this case) underestimate the actual level of support for Conservatives and UKIP.

Similarly, when asked about the choice for a party that they intend to make, respondents may name a party and are counted towards that party’s projected share of the vote. Some of these answers come from people who have firmly made up their mind, but for others this hides the fact that they are not yet certain about their choice. The polls that are currently conducted in the UK do not take into account the certainty with which respondents’ give their answers. This is particularly unfortunate in the case of a close race. We are in the dark about the proportions of voters who may still change their stated preference because they like another party almost as much as the one that they said they would vote for.

In a survey conducted by the British Election Study team in February 2010 (file feb10spb.dta), and which contains more detailed information than most election polls, we find that of those who unequivocally state that they will vote Conservative at the next general election, 4.5% actually hesitates between Conservative and Labour, and 7.6% hesitates between Conservative and Liberal Democrats. Of those who claim that they will support Labour 2.1% might still change to Conservative, and 13.8% to the Liberal Democrats. Of the projected Lib Dem support 8.9% might still go Conservative and 26.9% Labour. In all these situations, it will not take much for these voters to still change their vote intention because they are actually quite strongly attracted to two different parties at the same time.

When looking at the “don’t know’s”, we find that 12.7% is actually strongly attracted to the Conservatives, 27% to Labour, and 22.4% to the Liberal Democrats. When prodding the “don’t know’s” to indicate which party they lean to, we find that those leaning to Labour have a much stronger preference for that party than leaning Conservatives have for theirs. In other words, if, as is usually the case during election campaigns, the number of yet undecided voters declines as polling day draws nearer, Labour in particular and the Liberal Democrats to a smaller extent stand to gain more from this than the Conservatives.

The methodology employed here is analogous to that reported in Martin Kroh, Wouter van der Brug and Cees van der Eijk ‘Prospects for electoral change’, in Wouter van der Brug and Cees van der Eijk, European Elections and Domestic Politics – Lessons from the Past and Scenarios for the Future (University of Notre Dame Press, 2007, pp. 209-225).

If we want to gauge the possible dynamics in party support over this election campaign, we have to take into account the differences in the certainty with which respondents state their intended choices. In the current situation, doing so reveals an even closer race than most of the polls suggest.

Professor Cees van der Eijk