Estimating Ideal Points in the British House of Commons Using Early Day Motions

Abstract

This article develops a new method for estimating the ideological preferences of members of the British House of Commons. Existing methods produce implausible results due to high levels of party cohesion and strategic voting on the part of opposition parties. To circumvent these problems, this article estimates MP preferences using Early Day Motions (EDMs) as an alternative to roll‐call votes. The Bayesian ideal point model for the decision to sign an EDM takes into account both policy preferences and signing costs. The estimates obtained have greater face validity than previous attempts to measure preferences in the House of Commons, recovering the expected order of parties and of members within parties. The estimates successfully predict voting behavior in the House of Commons. As with other Bayesian ideal point methods, this approach produces natural uncertainty estimates and allows for easy calculation of quantities of interest such as member ranks.