title: Moderator analysis for subgroups with the metafor package
body: <p>I have a dataset with 2 moderators, <code>model</code> and <code>mechanism</code> which have 3 and 5 levels respectively. Separate meta-regression with each moderator using the <code>rma()</code> function in the <em>metafor</em> package revealed that both of them significantly influence the summary effect. However, fitting additive model with both moderators resulted in values for groups that do not exist, which i find difficult to interpret. So I decided to perform subgroup analysis for <code>model</code> with consequent meta-regression with the moderator <code>mechanism</code>. However, although I can run:</p>
  <pre><code>run.5 = rma(yi = es, vi = var, data = agg.es.mod, subset = model == &quot;IR&quot;)
  </code></pre>
  <p>I don't know how to run moderator analysis with <code>mechanism</code> for this subgroup. Is it possible at all to do this in <em>metafor</em>? Such function would be highly useful, because if not, I guess one must subdivide the data and run separate meta-analyses?</p>