How to divide the sum with the size in a pandas groupby
<p>I have a dataframe like</p>

<pre><code>  ID_0 ID_1  ID_2
0    a    b     1
1    a    c     1
2    a    b     0
3    d    c     0
4    a    c     0
5    a    c     1
</code></pre>

<p>I would like to groupby ['ID_0','ID_1'] and produce a new dataframe which has the sum of the ID_2 values for each group divided by the number of rows in each group.</p>

<pre><code>grouped  = df.groupby(['ID_0', 'ID_1'])
print grouped.agg({'ID_2': np.sum}), "\n", grouped.size()
</code></pre>

<p>gives</p>

<pre><code>           ID_2
ID_0 ID_1
a    b        1
     c        2
d    c        0
ID_0  ID_1
a     b       2
      c       3
d     c       1
dtype: int64
</code></pre>

<p>How can I  get the new dataframe with the np.sum values divided by the size() values?</p>