title: Simple slopes for interaction in Negative Binomial regression
body: <p>I am looking to obtain parameter estimates for one predictor when constraining another predictors to specific values in a negative binomial glm in order to better explain an interaction effect.</p>
  
  <p>My model is something like this:</p>
  
  <pre><code>model &lt;- glm.nb(outcome ~ IV * moderator + covariate1 + covariate2)
  </code></pre>
  
  <p>Because the <code>IV:moderator</code> term is significant, I would like to obtain parameter estimates for <code>IV</code> at specific values of <code>moderator</code> (i.e., at +1 and -1 SD). I can obtain slope estimates for <code>IV</code> at various levels of <code>moderator</code> using the <code>visreg</code> package but I don't know how to estimate SEs and test statistics. <code>moderator</code> is a continuous variable so I can't use the <code>multcomp</code> package and other packages designed for finding simple slopes (e.g., <code>pequod</code> and <code>QuantPsyc</code>) are incompatible with negative binomial regression. Thanks!</p>