title: How to reference a previous group for year-on-year calculations in a Crystal Report?
body: <p>I am writing a report which contains 3 groups: Year, Month and Supplier.</p>
  
  <p>Each page starts with the Year at the top, then each of the 12 months, with sales summaries for each. The user can then drill down to see the individual sales figures for each company from that month.</p>
  
  <p>e.g. the top level of my report looks like this:</p>
  
  <pre><code>2010  
     January    £20,000
     February   £30,000
     March      £15,000
     etc.
  
  2011
      January   £16,000
      February  £14,000
  </code></pre>
  
  <p>Now, I wish to compare the values of Jan 2011 with the same period of the previous year (in this case: 16,000 vs 20,000) so I can perform some additional calculations.</p>
  
  <p>Currently I am achieving this by using global variables called JanGlobal, FebGlobal, etc. which are stored and retrieved as necessary. This works because I know there will only be 12 months, so I can name the variables in advance.</p>
  
  <p>However, at the supplier level, I don't know how many suppliers there will be, or even if they will exist at all (suppliers might be new, so they won't have Year-on-year information; or they might have gone bust, and so don't exist in the new year).</p>
  
  <p>My suppliers drill down looks like this:</p>
  
  <pre><code>2010
      January
          Company 1    £5,000
          Company 2    £7,000
          Company 3    £8,000
  </code></pre>
  
  <p>So I need to compare a variable number of suppliers with their counterparts from 12 months previous, if they exist. Any ideas?</p>