UK OC OK? Interpreting Optimal Classification Scores for the U.K. House of Commons

Poole's (2000, Non-parametric unfolding of binary choice data. Political Analysis 8:211–37) nonparametric Optimal Classification procedure for binary data produces misleading rank orderings when applied to the modern House of Commons. With simulations and qualitative evidence, we show that the problem arises from the government-versus-opposition nature of British (Westminster) parliamentary politics and the strategic voting that is entailed therein. We suggest that political scientists think seriously about strategic voting in legislatures when interpreting results from such techniques.