A. Colin Cameron is Professor of Economics atthe University of California, Davis. He currently serves as Director of that university's Center on Quantitative Social Science Research. He is coauthor with Pravin Trivedi of Regression Analysis of Count Data. Pravin K. Trivedi is John H. Rudy Professor of Economics at Indiana University at Bloomington. He is on the editorial boards ofthe Econometrics Journal and the Journal of Applied Econometrics.