06 – Count Models

Categorical Data Analysis (Soc 681) · Purdue University · Fall 2026

The lecture notes for this section of the course, with the Stata and R code and output for every example. This site contains all the material and code for the course. If you prefer the slides in PDF format and/or the Stata replication do-file, these can be downloaded at: lecture slides · replication code.

Count outcomes

• What is a count variable?
  • Discrete
  • Values of zero or above
  • A summation of yes/no choices/outcomes
• E.g., Number of alcoholic drinks consumed last week; # of days attending religious services last month; # of protests attended in life; # of functional limitation; # of academic citations received
  • # of Prussian army deaths by horse kick (per year)

*Count von Count video

Contents

• 6.1 Explaining Count Distributions
• 6.2 The Poisson Regression Model
• 6.4 Poisson Regression Example
• 6.5 Assessing Models with Average Predictions
• 6.6 Negative Binomial Models
• 6.7 Another Negative Binomial Example
• 6.9 Zero Inflated Count Models
• 6.10 Model Specification
• 6.11 Plots of Predictions & MEs
• 6.12 Model Selection Tools
• 6.13 Review of Count DVs
• Appendix A6: Stata Code and Tips