🤖 AI Summary
This study addresses the incidental parameter problem arising from unit-specific fixed effects in nonlinear panel data models, particularly when the number of time periods per unit is small and conventional estimators break down. The authors propose a novel projection-based approach that eliminates these incidental parameters without imposing assumptions on the joint distribution of fixed effects and covariates. By constructing an identified set through an implementable correspondence between observables and unobserved heterogeneity, and leveraging random set theory together with moment inequalities, they develop a distribution-free partial identification framework. This framework accommodates both static and dynamic models as well as discrete and continuous outcomes, enabling robust inference even in short panels.
📝 Abstract
This paper introduces a new approach to econometric analysis of nonlinear panel data models when the number of observations per observational unit is small. In such models the presence of variables that are constant within, while varying across, units results in an incidental parameter problem. The approach taken in this paper removes these incidental parameters via projection, which produces a correspondence specifying all combinations of observed variables and within-unit-varying unobserved heterogeneity that are achievable by choice of some value of the unit-specific incidental parameters. With unit-specific variables removed, there is no need for assumptions concerning their joint distribution with other variables. The result is an incomplete model which is typically partially identifying. Identified sets are characterized via moment inequalities using tools of random set theory. Examples of application to static and dynamic models with discrete or continuous outcomes using distribution-free restrictions on within-unit-varying unobserved heterogeneity are presented.