Identifying Intra-Party Voting Blocs in UK House of Commons

Abstract

Legislative voting records are an important source of information about legislator preferences, intra-party cohesiveness, and the divisiveness of various policy issues. Standard methods of analyzing a legislative voting record tend to have serious drawbacks when applied to legislatures, such as the UK House of Commons, that feature highly disciplined parties, strategic voting, and large amounts of missing data. We present a method (based on a Dirichlet process mixture model) for analyzing such voting records that does not suffer from these same problems. Our method is model-based and thus allows one to make probability statements about quantities of interest. It allows one to estimate the number of voting blocs within a party or any other group of MPs. Finally, it can be used as both a predictive model and an exploratory model. We illustrate these points through an application of the method to the voting records of Labour Party MPs in the 1997-2001 session of the UK House of Commons.