How do you find the average of a sum? To explain..

 

Invoice Number      Value                  Line Item

1001                      $15                           1

1001                      $150                         2

                       SUM=$165

 

2001                      $25                           1

2001                      $15                           2

                       SUM=$40

 

If I were to simply average the value, I would get (15+150+25+15) / 4. Which would mean a average of 51.25.

 

However, in the case I am working on, this is bad as there are 4-5 line items per invoice. One line item in the thousands, the other 3-4 at meer dollars. So I want to sum the invoice, then average the sums of the invoices. This would yeild (165+40) / 2. For a real invoice average of 102.5.

 

How do I go about only getting the average of the sums of invoices?

 

If I need to be more clear, let me know. But please respond asap.

 

Thanks!

 

-Chris