title: Pandas : compute mean or std (standard deviation) over entire dataframe
body: <p>Here is my problem, I have a dataframe like this :</p>
  
  <pre><code>    Depr_1  Depr_2  Depr_3
  S3  0   5   9
  S2  4   11  8
  S1  6   11  12
  S5  0   4   11
  S4  4   8   8
  </code></pre>
  
  <p>and I just want to calculate the mean over the full dataframe, as the following doesn't work :</p>
  
  <pre><code>df.mean()
  </code></pre>
  
  <p>Then I came up with :</p>
  
  <pre><code>df.mean().mean()
  </code></pre>
  
  <p>But this trick won't work for computing the standard deviation.  My final attempts were :</p>
  
  <pre><code>df.get_values().mean()
  df.get_values().std()
  </code></pre>
  
  <p>Except that in the latter case, it uses mean() and std() function from numpy. It's not a problem for the mean, but it is for std, as the pandas function uses by default <code>ddof=1</code>, unlike the numpy one where <code>ddof=0</code>.</p>