Forecasting the 2010 general election using aggregate local election data

The paper presents a revised method for estimating national vote shares using aggregate data from local government by-elections. The model was originally developed to forecast the annual outcome of local elections but was adapted in time to provide an accurate forecast of Labour’s landslide victory at the 1997 general election. However, over the past decade the changing pattern of party competition which has seen parties becoming more selective about which elections to contest has led to more elections being excluded from the modelling because they failed to meet the exacting criteria that all three major parties, Conservative, Labour and Liberal Democrats, had contested both the by-election and the previous main election, normally held in May. Relaxing these criteria, although increasing the number of available cases would adversely affect the forecast, over- or under-estimating party votes. Instead, the revised method overcomes the problem of differential competition by estimating vote shares for parties that contest one but not both elections. A further innovation is the calculation of a weighted moving quarterly average which takes account of the number of days elapsed between the by-election date and the date of forecast. Using the new method we provide estimates for likely party shares for the 2010 general election.