title: How do I drop a factor level with no observations?
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/1195826/dropping-factor-levels-in-a-subsetted-data-frame-in-r">dropping factor levels in a subsetted data frame in R</a>  </p>
  </blockquote>
  
  
  
  <p>I have a data frame with several variables that I'm running a mixed model on using <code>lme()</code>.  One of the variables, ForAgeCat, has five factor levels: 1,2,3,4,5. </p>
  
  <pre><code>str(mvthab.3hr.fc$ForAgeCat)
   &gt;Factor w/ 5 levels "1","2","3","4",..: 5 5 5 5 5 5 5 5 5 5 ...
  </code></pre>
  
  <p>The problem is that factor level 3 actually doesn't exist, that is, in this dataset (which is a subset of a larger dataset) there are no observations from factor level 3, which I think is messing with my modeling in lme().  Can someone help me to remove/eliminate factor level 3 from the list of factor levels?</p>