An Ecological Item-Response Model for Multiple Subsets of Respondents with Application to the European Court of Justice

The European Court of Justice (ECJ) has fostered the development of a common European legal order, and in doing so, has asserted itself and its supremacy more, and more successfully, than any other international court. It has maintained features of international courts such as its composition of one judge per member state, while employing other tools of national high courts such as en banc decisions and organization into chambers, that together hide internal dissent and shield the ECJ from direct monitoring or curbing by the member states. The same shield has frustrated efforts to quantify the court's responsiveness to member states, with limited evidence that the ECJ yields to some member-state interest some of the time, but nonetheless has advanced integration beyond national governments' wishes. This equivocation arises at least in part from a failure to include relevant information about the court's composition and organization.In fact, the six-year renewable terms of judges, their previous qualifications and affiliations, and the internal organization into chambers all provide prior information that can and should be incorporated into a more complete model of judicial behavior. I develop an extension of the well-studied item-response model to infer judges' preferences, using the structured ecological data from the cases they heard and relevant prior information about judges and the national governments that appoint them as well as information about cases. I offer new, more rigorous tests for existing theoretical hypotheses about the ECJ's deference to certain actors and preference for integration. The model is applicable to other settings of structured ecological data. Many other national and international courts hear cases in subset chambers, and relevant prior information should be included rather than ignored in models of judicial behavior.