title: Pandas groupby => AttributeError: 'function' object has no attribute 'mean'
body: <p>I try to compute the average size of a population composed of men and women:</p>
  <pre><code>&gt;&gt;&gt; import pandas as pd
  &gt;&gt;&gt; df = pd.DataFrame([[175 , 'male' ], [181 , 'male' ], [165 , 'female' ], [179 , 'male' ], [156 , 'female' ]], columns=['size', 'sex'])
  &gt;&gt;&gt; df.head()
  
      size    sex
  0   175     male
  1   181     male
  2   165     female
  3   179     male
  4   156     female
  </code></pre>
  <p>I would like to use Pandas groupby method to compute the average. Therefore, the following command returns an error:</p>
  <pre><code>&gt;&gt;&gt; df.groupby('sex').size.mean()
  ---------------------------------------------------------------------------
  AttributeError                            Traceback (most recent call last)
  &lt;ipython-input-17-a31e7f02b69d&gt; in &lt;module&gt;()
  ----&gt; 1 df.groupby('sex').size.mean()
  
  AttributeError: 'function' object has no attribute 'mean'
  </code></pre>