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Abstract
This paper outlines several methods for forecasting the next British general election on a daily basis from 20 months prior using opinion polls. It discusses their performance for previous electoral cycles and shows that the two models with the best historical record lead to substantially different predictions for 2015, but they can be averaged. The historical relationship between the polls and the vote suggests that government support rises substantially in the run up to elections, that Conservatives outperform but Labour underperform relative to the polls, and parties generally recover from low points or decline from high ones. Approximate prediction intervals and probabilities for key events are also generated. Despite the Conservatives trailing in the polls by 7 points in early October 2013, the models suggest a substantial Conservative lead at the 2015 election with a 64% chance of being the largest party and 42% chance of an overall majority. The estimated probability of a hung parliament (40%) is instructive for understanding the operation of the electoral system.
Acknowledgements
I am very grateful for comments on previous drafts to John Curtice, Jouni Kuha, Matt Lebo, Clive Payne, Mark Pickup, Chris Prosser and three anonymous reviewers, and to Rob Ford, Will Jennings and Chris Wlezien for early discussions about this work. Most especially I am indebted to Will Jennings for compiling and preparing the polling data, including interpolation and correction for overlaps, as used in Wlezien et al. (2013). Finally I am also very grateful for the numerous and wide ranging comments on Twitter and elsewhere online in response to previous drafts. The paper has been revised substantially in part thanks to them.
Notes
1 This only reflects the time this work was started relative to the 2015 election.
2 This is a technical term is used just to refer to the difference between the average poll and the election result. It is not necessarily the product of any methodological or other choice by pollsters and may not be possible to predict, control or correct for and certainly there is not any accusation here of politically motivated interference.
3 I have added 96 polls that were missing from the Wlezien et al. (Citation2013) data set, all of them from the final three months of election cycles up to February 1974. Also note that allocations of fieldwork midpoints involved rounding down not rounding up as was mistakenly reported in Wlezien et al. (Citation2013).
4 Government recoveries can be engineered by a change of leadership as when John Major replaced Margaret Thatcher 15 months before the 1992 election. Douglas-Home replaced Macmillan a year before the 1964 election, primarily because of illness but arguably also because of unpopularity. But the only other two changes, from Wilson to Callaghan and from Blair to Brown, were more than 20 months prior to the election and did not improve their party's standing.
5 The 2015 general election is expected regardless of the outcome of the referendum on Scottish independence.
6 Since this is the first time the Liberals have been in government since the war, there is no evidence as to how this affects the likelihood of recovery before the election. One could look at the experience of the 1979 minority administration supported by the Liberals to argue that a recovery is likely, but this is too limited an evidence base to estimate the magnitude of the effect.
7 That paper provided two forecasts – one from just before the campaign and one from the final week of the campaign. The pre-campaign forecast was adjusted for the historical polls–vote relationship (stage 2 of the method) but the final forecast was not (given the nature of the regressions run being close to identity). Note that the pre-campaign polls–vote relationship was estimated using polls aggregated over a week before the campaign (or equivalent) for each cycle.
8 There are various mechanisms which might cause the pattern of increasing strength of the coefficients, such as vote intention increasingly based on party identification or leadership ratings, or just the diminishing time left for change to occur. See Wlezien et al. (Citation2013) for graphs and discussion of the evolution of the coefficients from similar regressions.
9 That is, Σabs(predicted p − actual p)/2, with summation over p = Con, Lab, Lib, Oth.
10 In truth, there is perhaps the possibility of developing a theory of when opinion-turning events are most likely to occur, e.g. at party conferences and Budget speeches, and which periods are likely to be disproportionately quiet, e.g. holidays. Developing this idea would require too much groundwork for this paper and various informal analyses suggest that there is limited systematic lasting legacy of budgets and conferences.
11 I assume no change in vote share for the Scottish National Party (SNP) and Plaid Cymru (PC). Otherwise, I assume that the largest minor party in the constituency in 2010 will get all the increase or decrease in the Other share of the vote. In many cases, this will be UKIP. I further assume that the Green's will hold their seat, that Wyre Forest will stay Conservative (after they narrowly defeated an independent incumbent in 2010) and that the speaker's seat will remain Conservative.
12 In practice, the prediction intervals for seats are slightly skewed and doubtless a full simulation exercise would not produce a density which was perfectly normal, but the approximation seems good enough for the purpose given that with a fixed estimate for the Liberal Democrat and Other party share the variation in seats is overwhelmingly between Conservative and Labour and the probability calculations are relatively insensitive to the estimate of the standard deviation within a reasonable range.
13 The technology can be used to generate forecasts for different scenarios so predictions from different poll averages can be compared.
14 Not including SNP or PC.
15 Approaching the last election, a hung parliament was more likely than not, but this was not reflected in the media debate. Things may be different now that there is hung parliament with a coalition, but it could still be seen as an aberration or people may become too inclined to think it will happen again.
16 Except that perversely the prediction intervals for Labour are much narrower as result of the smaller more appropriate post-74 sample.
17 See Wlezien et al. (Citation2013) for information on the variation in the polls and how it differs between parties and both within and across cycles.