First you need to define a Group summary type for the field you want to use to group results. The summary results page displays a list of values for this grouped field. You can click a value to drill down each group of results.
After you have defined a Group, you can define another summary type to provide quantitative results on the summary results page:
Count -Counts the number of results for the field. Counts are usually defined on name or ID fields to get a count of records for each group.
Sum -Sums values for the field.
Minimum -Displays the minimum amount, or the earliest date value.
Maximum -Displays the maximum amount, or the most recent date value.
Average -Calculates the average value for the field.