Why Did the Polls Overestimate Liberal Democrat Support? Sources of Polling Error in the 2010 British General Election

Pollsters once again found themselves in the firing line in the aftermath of the 2010 British general election. Many critics noted that nearly all pollsters in 2010 expected a substantial surge for the Liberal Democrats that did not materialize. Basing conclusions regarding the relative merits of pollsters or benefits of methodological design features on inspection of just the final poll from each pollster is inherently problematic, because each poll is subject to sampling error. This paper uses a state-space model of polls from across the course of the 2010 election campaign which allows us to assess the extent to which particular pollsters systematically over-or under-estimate each main party's share of the vote, while allowing for both the usual margins of error for each poll and changes in public opinion from day-to-day. Thus, we can assess the evidence for systematic differences between pollsters' results according to the use of particular methodologies, and estimate how much of the discrepancy between the final polls and the election outcome is due to methodological differences that are associated with systematic error in the polls. We find robust evidence of an over-estimation in Liberal Democrat support, but do not find evidence to support the hypothesis that the polls erred due to a late swing away from the party, nor that any of the methodological choices made by pollsters were significantly associated with this over-estimation.