Since Regression Analysis of Count Data was published in 1998, significant new research has contributed to the range and scope of count data models. This growth is reflected in many new journal articles, fuller coverage in textbooks, and wide interest in and availability of software for handling count data models. These developments (to which we have also contributed) have motivated us to revise and expand the first edition. Like the first edition, this volume reflects an orientation toward practical data analysis.

The revisions in this edition have affected all chapters. First, we have corrected the typographical and other errors in the first edition, improved the graphics throughout, and where appropriate we have provided a cleaner and simpler exposition. Second, we have revised and relocated material that seemed better placed in a different location, mostly within the same chapter though occasionally in a different chapter. For example, material in Chapter 4 (generalized count models), Chapter 8 (multivariate counts), and Chapter 13 (measurement errors) has been pruned and rearranged so the more mainstream topics appear earlier and the more marginal topics have disappeared altogether. For similar reasons bootstrap inference has moved to Chapter 2 from Chapter 5. Our goal here has been to improve quality of synthesis and accessibility of material to the reader. Third, the final few chapters have been reordered. Chapter 10 (endogeneity and selection) has moved up from Chapter 11.