Changing summary fields
You can change summary fields; for example, to change the type of summary and the field you want to group data by.
If you change the type of summary calculation, FileMaker Pro recalculates the value for every record in the database when you exit the Manage Database dialog box.
To change a summary field:
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With the database open, choose File menu > Manage > Database.
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To change a summary field in Table View:
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Right-click the column heading for the summary field and choose Field > Field Options from the shortcut menu.
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Related topics 
Defining summary fields
Summary data is missing or incorrect
Specifying formats for fields containing numbers