title: model.matrix generates fewer rows than original data.frame
body: <p>Why doesn't a model matrix necessarily have the same number of rows as the data frame?</p>
  
  <pre><code>mergem = model.matrix(as.formula(paste(response, '~ .')), data=mergef)
  dim(mergef)
  # [1] 115562     71
  dim(mergem)
  # [1] 66786   973
  </code></pre>
  
  <p>I tried looking for hints in the documentation but couldn't find anything.  Thanks in advance.</p>