Using PREVGROUPVAL in Report #inSalesforce

Using this, the value of a specified previous grouping will be returned.

You have to choose the grouping level and increment like in below syntax. The increment is the number of columns or rows before the current summary. The default is 1; the maximum is 12. 

You can use this function only in custom summary formulas and at grouping levels for reports, but not at summary levels.

Syntax : PREVGROUPVAL(summary_field, grouping_level [, increment])

Here summary_field is the name of the grouped row or column, grouping_level is the API name of the peer level group whose summary value for the previous grouping is used, and increment is the number of groupings previous.

Scenario:

Suppose we have report which is an opportunity matrix with columns grouped by Close Date and rows by Stage.

and if we have to find how much the Amount changed compared to previous year : AMOUNT:SUM - PREVGROUPVAL(AMOUNT:SUM, CLOSE_DATE)

This calculates, for each year, the difference in amount from the previous year shown in the report

and to display the above in percentage changed compared to previous : (AMOUNT:SUM - PREVGROUPVAL(AMOUNT:SUM, CLOSE_DATE))/ PREVGROUPVAL(AMOUNT:SUM, CLOSE_DATE)