title: Robumeta package - error when fitting a model with an interaction [R, Robumeta, meta-aanlysis]
body: <p>I am meta-analyzing using the 'robumeta' package.
  When I fit a model with an interaction, sometime I receive an error and could not analyze. </p>
  
  <pre><code>res2 &lt;- robu (yi ~  fix1 * fix2,
               data = dat, studynum = study, 
  var.eff.size = vi, modelweights = "CORR", 
  rho = 0.8, small = T)
  </code></pre>
  
  <blockquote>
    <p>Error in solve.default(sumXWX) :    Lapack routine dgesv: system is
    exactly singular: U[9,9] = 0</p>
  </blockquote>
  
  <p>How can I cope with this? Thank you.</p>