title: Pandas - Propagating variance
body: <p>For data of the form</p>
  
  <pre><code>mean            var             count
  31.5910645161   747.570011484   310
  45.7            350.0658        2
  77.2548205128   4968.46005809   195
  166.830361446   13755.5734253   166
  40.29           208.8968        2
  254.35          15204.1922      2
  4.81            0.0             1
  56.0124200913   962.697805171   1533
  114.25          0.0             1
  24.12           422.257129412   18
  </code></pre>
  
  <p>Where there a many more repetitions of count later. I need to <code>groupby('count').agg('mean','var')</code> in order to propagate the variance properly. However, that code does not work (mean and var don't know what to do with the 2 columns), and of course just using mean is out of the question (the mean of the variance is not the variance of the mean). How do you do this such that the variance gets sent forward properly?</p>