title: pandas calculating a shifted rolling variance by group
body: <p>I have a pandas-dataframe holding a GROUP, DATE, VALUE and VARIANCE column:</p>
  
  <pre><code>Index      GROUP   DATE         VALUE    VARIANCE
  1          g1      2015-12-02   10       3.2                    
  2          g1      2015-10-12   9        4.25                  
  3          g1      2013-12-13   8        8               
  4          g1      2013-12-13   11       8
  5          g1      2013-07-15   7        NaN
  6          g1      2015-12-02   11       NaN                   
  7          g2            ...
  </code></pre>
  
  <p>Basically I want to calculate the shifted rolling variance for the VALUE column. So the value for variance for Index 1 is the variance over the values in Index 2-6, etc.</p>
  
  <p>My first approach was to use an expanding windown to calculate the variances and shift the values by 1 but I am not sure whether this is the right approach here. I am happy about every suggestion.</p>