Predictable and Unpredictable Changes in Party Support: A Method for Long-Range Daily Election Forecasting from Opinion Polls

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.