Bias in instrumental-variable estimators of fixed-effect models for count data

This note looks at the properties of instrumental-variable estimators of models for non-negative outcomes in the presence of individual effects. We show that fixed-effect versions of the estimators of Mullahy (1997) and Windmeijer and Santos Silva (1997) are inconsistent under conventional asymptotics, in general, and that inference based on them in long panels requires bias correction. Such corrections are derived and their effectiveness is investigated in numerical experiments. Consistent estimation in short panels is nonetheless possible in the setting underlying Mullahy's (1997) approach using a differencing strategy along the lines of Wooldridge (1997) and Windmeijer (2000). (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

This note examines the properties of instrumental-variable estimators in models for non-negative outcomes with individual effects. The study shows that fixed-effect versions of the estimators are generally inconsistent under conventional asymptotics and require bias correction for inference in long panels. The effectiveness of these corrections is investigated through numerical experiments. However, consistent estimation in short panels is possible using a differencing strategy based on previous approaches.