title: lrm function from rms package does not function
body: <p>I need to cluster the standard errors by dyads, and I am trying to use the rms package. To do so I need to fit my model using lrm instrad of glm. When I type <code>library(rms)</code> I get the following error:</p>
  
  <pre><code>Error in loadNamespace(i, c(lib.loc, .libPaths()), versionCheck = vI[[i]]) : 
    there is no package called ‘mvtnorm’
  Error: package or namespace load failed for ‘rms’,
  </code></pre>
  
  <p>and when I try to use the following comand to cluster the standard errors by dyads I get the error that the function lrm does not exist. </p>
  
  <pre><code>fit=lrm(disease ~ age + study + rcs(bmi,3), x=T, y=T, data=dataf)
  fit
  robcov(fit, cluster=dataf$id)
  bootcov(fit,cluster=dataf$id)
  </code></pre>
  
  <p>and also does anyone know what <code>x=T</code> and <code>y=T</code> are? I have just started working in R and with statistics in general, so none of this is quite obvious to me. </p>