title: pandas get column average/mean
body: <p>I can't get the average or mean of a column in pandas. A have a dataframe. Neither of things I tried below gives me the average of the column <code>weight</code></p>
  
  <pre><code>&gt;&gt;&gt; allDF 
           ID           birthyear  weight
  0        619040       1962       0.1231231
  1        600161       1963       0.981742
  2      25602033       1963       1.3123124     
  3        624870       1987       0.94212
  </code></pre>
  
  <p>The following returns several values, not one:</p>
  
  <pre><code>allDF[['weight']].mean(axis=1)
  </code></pre>
  
  <p>So does this:</p>
  
  <pre><code>allDF.groupby('weight').mean()
  </code></pre>