title: mean() warning: argument is not numeric or logical: returning NA
body: <p>I have a data frame with two columns. When I try to calculate <code>mean</code>, I get this message: </p>
  
  <pre><code>[1] NA
  Warning message:
  In mean.default(results) : argument is not numeric or logical: returning NA`
  </code></pre>
  
  <p>where 'results' is my data set. Any advice on getting around this problem?</p>