title: Month to month data comparison
body: <p>I'm trying to create a formula in Crystal Reports using the below data that will calculate the difference of DxLoaded in the current RunDt minus the previous month RunDt for each of the two Data Sources.  Each month a new RunDt will be populated.  When the report is refreshed I need to have the formula calculate using the most current RunDt comparing it to the previous RunDt for each Data Source.</p>
  
  <p>For example, I would like to calculate 5,491,932 for the 1203 RunDt minus 3,830,842 for the 1202 RunDt.  Then have the formula do that for each Data Source that might be populated.  There will also be service year of 2012 for the same Data Source and would like 2011 compared only with 2011 and 2012 with 2012.  I've tried using the previous function but haven't had luck with obtaining the correct results.</p>
  
  <pre><code>Yr             Data_Source  RunDt_YYMM      DxLoaded   
  2011           ABS          1203            5,491,932  
  2011           ABS          1202            3,830,842  
  2011           IALT         1203            9,193,144  
  2011           IALT         1202            6,578,678  
  2012           ABS          1203            1,837,900  
  2012           ABS          1202            1,083,124  
  2012           IALT         1203            4,223,111  
  2012           IALT         1202            2,985,543  
  </code></pre>
  
  <p>Any help of suggestions are greatly appreciated!!</p>
  
  <p>Thank you!</p>