Anchors Away: A New Approach for Estimating Ideal Points Comparable across Time and Chambers

Abstract

Existing methods for estimating ideal points of legislators that are comparable across time and chambers make restrictive assumptions regarding how legislators' ideal points can move over time, either by fixing some legislators' ideal points or by constraining their movement over time. These assumptions are clearly contradictory to some theories of congressional responsiveness to election dynamics and changes in constituency. Instead of using legislators as anchors, our approach relies on matching roll calls in one chamber and session with roll calls or cosponsorship decisions on identical bills introduced in a different chamber or session. By using these “bridge decisions” to achieve comparability, we can remove any assumptions about the movement of legislators' ideal points. We produce these estimates for both chambers from the 102nd (1991–92) to 111th (2009–11) Congresses, and we show that our estimates provide interesting insights into the nature of legislative behavior change.