PREVGROUPVAL is a function used in the reports custom summary formulas to return the value of a summary field from a previous grouping.
PREV GROUP VAL stands for Previous Grouping Value.
This function is often used to analyze the trend of groupings in a report, often visualized in a line chart.
It is also possible to use PREVGROUPVAL to calculate the percent change of one period versus another.
Example: In a report where the opportunities gained per week are displayed, with PREVGROUPVAL function it is possible to calculate and visualize a three-week moving average of opportunity amounts:
(Oppty Sum +
PREVGROUPVAL (Oppty sum, 1) +
PREVGROUPVAL (Oppty sum, 2) ) / 3