title: Standard deviation for the difference of two dataframes with group by
body: <p>I have two panda DataFrames:</p>
  
  <p>Dataframe Yahoo:</p>
  
  <pre><code>date        ticker  return
  2017-01-03  CRM     0.018040121229614625
  2017-01-03  MSFT    -0.0033444816053511683
  2017-01-04  CRM     0.024198086662915008
  2017-01-04  MSFT    -0.0028809218950064386
  2017-01-05  CRM     -0.0002746875429199269
  2017-01-05  MSFT    0.0017687731146487362
  </code></pre>
  
  <p>Dataframe Quandl:</p>
  
  <pre><code>date        ticker  return
  2017-01-03  CRM     0.018040120991250852
  2017-01-03  MSFT    -0.003344466975803595
  2017-01-04  CRM     0.024198103213211475
  2017-01-04  MSFT    -0.0028809268004892363
  2017-01-05  CRM     -0.00027464144673694513
  2017-01-05  MSFT    0.0017687829680113065
  </code></pre>
  
  <p>I would like to get the standard deviation for the difference of Yahoo’s and Quandl’s 'return' data calculated across all ticker symbols for each day and data field. </p>
  
  <p>How can I get that?</p>