The British Election Study 2015 general election constituency forecast

Abstract

This paper develops a votes-to-seats nowcast model using individual level data from the British Election Study Internet panel to model the flow of the vote between 2010 and 2015. Transition matrices for each constituency are calculated using multinomial models of flows between 2010 origins and 2015 destinations. Respondents are reweighted to the polling average for England, Scotland and Wales and are further reweighted using predicted turnout to downweight respondents who are less likely to vote. The forecast performed poorly in 2015 because of polling errors and because the “Ashcroft” constituency vote intention question overestimated the Liberal Democrat incumbency effect. Without these errors, the forecast would have come much closer to the result but would still have underestimated the Conservatives' seats.