One of the key features of any report is the ability to display summary, or aggregate, information. For example, a sales report can show the overall sales total; sales subtotals by product type, region, or sales representatives; average sales figures; or the highest and lowest sales figures.
Aggregating data involves performing a calculation on a set of values rather than on a single value. For a simple listing report, aggregate calculations are performed on values in a specific field, over all the data rows in the report.
For example, BIRT calculates an average payment in a report by adding the values in the Amount field in every row, then dividing the total by the number of rows.