Whenever you create a summary report or provide summary information, such as totals; it is good policy to make it traceable back to the detail(s) that make up the summary. For example, for web-based reports that show sub-totals, make each sub-total amount a hyperlink that sends the user to a detail report that shows the details that make up the summary.

This is not only good user-interface design, but may prevent a lot of phone calls to you resembling, "Where does this number come from?"
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