Try this: About Summary Functions
Summary functions let you use grouping values in custom summary formulas for reports. Formulas are only supported for summary and matrix reports.
There are two summary functions: PARENTGROUPVAL and PREVGROUPVAL.
PARENTGROUPVAL
Use this function to calculate values relative to a parent grouping.
Description: This function returns the value of a specified parent grouping. A “parent” grouping is any level above the one containing the formula. You can only use this function in custom summary formulas for reports. Use: Summary: PARENTGROUPVAL(summary_field, grouping_level)
Matrix: PARENTGROUPVAL(summary_field, parent_row_grouping, parent_column_grouping)
Where summary_field is the summarized field value, grouping_level is the parent level for summary reports, and parent_row_level and parent_column_level are the parent levels for matrix reports.
Example:
TOTAL_PRICE:SUM/PARENTGROUPVAL(TOTAL_PRICE:SUM, GRAND_SUMMARY)
This formula calculates, for each product, its relative size compared to the grand total. In this example, the report is a summary of opportunities and their products, grouped by Product Name.
PREVGROUPVAL
Use this function to calculate values relative to a peer grouping. If there is no previous grouping, the function returns a null value.
Description: This function returns the value of a specified previous grouping. A “previous” grouping is one that comes before the current grouping in the report. Choose the grouping level and increment. The increment is the number of columns or rows before the current summary. The default is 1; the maximum is 12. You can only use this function in custom summary formulas for reports. Use:
PREVGROUPVAL(summary_field, grouping_level [, increment])
Where summary_field is the name of the grouped row or column, grouping_level is the summary level, and increment is the number of rows or columns previous.
Example:
AMOUNT:SUM - PREVGROUPVAL(AMOUNT:SUM, CLOSE_DATE)
This formula calculates, for each month, the difference in amount from the previous month shown in the report. In this example, the report is an opportunity matrix with columns grouped by Close Date and rows by Stage.
