title: Standard deviation and mean of complete pandas dataframe
body: <p>I've got quite a large dataset and would like to calculate the mean and the standard deviation, across all columns and rows. Unfortunately, I haven't got a proper solution for this yet.
  My dataset looks a littlebit like that (total of 600 rows):</p>
  
  <p><a href="https://i.stack.imgur.com/1lSjE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/1lSjE.png" alt="&lt;code&gt;df.head()&lt;/code&gt; of dataset"></a></p>
  
  <p>When I use the pandas-function <code>weekl_rtr.mean()</code> I just calculate the mean across each column. The workaround, that might work for the mean should be <code>weekl_rtr.mean().mean()</code>, but this does not work for the standard deviation.
  Do you have an idea, how to solve this?</p>
  
  <p>Thank you and kind regards,</p>
  
  <p>Markus</p>