A. Colin Cameron is Professor of Economics at the University of California, Davis. He currently serves as Director of that university's Center on Quantitative Social Science Research. He has also taught at the Ohio State University, and held short-term visiting positions at Indiana University at Bloomington and at a number of Australian and European universities. His research in microeconometrics has appeared in leading econometrics and economics journals. He is coauthor with Pravin Trivedi of Regression Analysis of Count Data (Econometric Society Monograph series No. 30, Cambridge University Press). Pravin K. Trivedi is John H. Rudy Professor of Economics at Indiana University at Bloomington. He has also taught at The Australian National University and University of Southampton, and has held short term visiting positions at a number of European universities. His research in microeconometrics has appeared in most leading econometrics and health economics journals. He coauthored Regression Analysis of Count Data with Colin Cameron and is on the editorial boards of the Econometrics Journal and the Journal of Applied Econometrics.