For certain files it is possible to add up particular fields in the highlighted records and display the total on the screen. This is done using the Sum Selection command.
Highlight the records to be added
Choose Select>Sum Selection or press Ctrl-=/⌘-=
The highlighted records are summed and the result is displayed.
Tip: On the Mac, if you hold down the option key, the Close button will change to Keep, enabling you to keep this sum around and do another.
