title: Counting unique items in data frame
body: <p>I want a simple count of the number of subjects in each condition of a study. The data look something like this:</p>
  
  <pre><code>subjectid  cond   obser variable
  1234        1        1      12   
  1234        1        2      14
  2143        2        1      19
  3456        1        1      12 
  3456        1        2      14 
  3456        1        3      13   
  
  etc       etc    etc       etc
  </code></pre>
  
  <p>This is a large dataset and it is not always obvious how many unique subjects contribute to each condition, etc. </p>
  
  <p>I have this in a data.frame.</p>
  
  <p>What I want is something like</p>
  
  <pre><code>cond   ofSs 
  1       122 
  2        98
  </code></pre>
  
  <p>Where for each "condition" I get a count of the number of unique Ss contributing data to that condition. Seems like this should be painfully simple. </p>