title: Finding the mean and standard deviation of a timedelta object in pandas df
body: <p>I would like to calculate the <code>mean</code> and <code>standard deviation</code> of a <code>timedelta</code> by bank from a <code>dataframe</code> with two columns shown below. When I run the code (also shown below) I get the below error:</p>
  
  <pre><code>pandas.core.base.DataError: No numeric types to aggregate
  </code></pre>
  
  <p>My dataframe:</p>
  
  <pre><code>   bank                          diff
     Bank of Japan                 0 days 00:00:57.416000
     Reserve Bank of Australia     0 days 00:00:21.452000
     Reserve Bank of New Zealand  55 days 12:39:32.269000
     U.S. Federal Reserve          8 days 13:27:11.387000
  </code></pre>
  
  <p>My code:</p>
  
  <pre><code>means = dropped.groupby('bank').mean()
  std = dropped.groupby('bank').std()
  </code></pre>