title: Groupby values in one column the result of dividing the sum of column b by the sum of column A
body: <p>I have a data frame with multiple columns and thousands of rows.  One column contains segmentation classification (ScoreSeg).  Other columns contain different values related to a item in that row.  I would like to groupby on 'ScoreSeg', (the sum of column 'PredMbrs' ) / (sum column b: 'EstMbrs')</p>
  
  <p>I tried the following:</p>
  
  <pre><code>pr.groupby('ScoreSeg')['PredMbrs'].sum()/pr['EstMbrs'].sum()
  </code></pre>
  
  <p>Actual results are:</p>
  
  <pre><code>ScoreSeg
  1-3     0.131708
  4-5     0.060665
  6-6     0.609900
  7-8     0.167228
  9-11    0.100284
  </code></pre>
  
  <p>Name: PredMbrs, dtype: float64</p>
  
  <p>The Expected results are:</p>
  
  <pre><code>1-3 0.954568997
  4-5 1.011131346
  6-6 1.056760686
  7-8 1.176788844
  9-11    1.211452952
  </code></pre>