# how to interpret interaction effects in negative binomial model

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## how to interpret interaction effects in negative binomial model

 Dear all, I have a question about how to interpret the interaction items in negative binomial regression. In the following model “post” is a dummy variable (0 or 1) to indicate two different periods (0 represents the first period, 1 represents the second period).  “treatment” is a dummy variable (0 or 1) to indicate two different groups –“treatment sample”(1) vs. “control sample” (0). The interaction is the product of the two dummies. The dependent variable is the number of analysts. My research objective is to examine whether the number of analysts changes over the two periods, and whether the changes over periods differ between the treatment sample and control sample. I have the following questions for the estimates below: (1) the coefficient on "post" is not significant, does this mean that the change in the number of analysts from period 1 to period2 is not statistically significant in the control group? (2) the coefficient on the interaction term "post*treatment" is significantly positive, does this mean that the change in the number of analysts from period 1 to period2 is significantly greater in the treatment sample than the control sample? How to interpret the coefficient on the interaction term exactly? How can I calculate if the changes in number of analysts from period 1 to period 2 differ between the treatment sample and control sample? Negative binomial regression                   Number of obs   =      30274 Dispersion = mean                               Wald chi2(37)   =          . Log pseudolikelihood = -27412.392              Prob > chi2     =          .                                                                                                             (Std. Err. adjusted for 45 clusters in n) ---------------------------------------------------------------------------            |             Robust  Analysts  |    Coef.    Std. Err.     z     P>|z|   [95% Conf. Interval] -----------+------------------------------------------------------------- post          .0610886  .0743914     0.82   0.412   -.0847159    .2068931 treatmen     -2.975135  .1591135   -18.70   0.000   -3.286992   -2.663278 post*treatment .214007  .0730457     2.93   0.003    .0708402    .3571739 --------------------------------------------------------------------------- Your help is greatly appreciated. -- Shiheng Wang Assistant Professor Department of Accounting School of Business and Management Hong Kong University of Science and Technology Tel: 852 2358 7570 Fax: 852 2358 1693 Email: [hidden email] * *   For searches and help try: *   http://www.stata.com/help.cgi?search*   http://www.stata.com/support/statalist/faq*   http://www.ats.ucla.edu/stat/stata/