Identifying Intraparty Voting Blocs in the U.K. House of Commons

Legislative voting records are an important source of information about legislator preferences, intraparty 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 United Kingdom 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 members of parliament (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 U.K. House of Commons.