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Gradebook categories are good to use when trying to organize a complex gradebook or set different aggregations (i.e., extra credit) to a group of grade items.
To add a new category goto the Gradebook Setup.
Scroll to the bottom and click Add category.
Fill go to the Grades from the top level navigation, then click Gradebook setup from the left side drop-down. Fill out the new category form name and click "Save changes". In most circumstances, you can leave settings at the default such as Natural Aggregation.
Coursesite will recalculate your grades. Click continue to return to the Gradebook Set-up.
The new category will be added to the bottom of the categories and items view.
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Tip: You can set a category to zero weight if needed - example if you just want to check a student paper for plagiarism with a Turnitin assignment but do not want to grade electronically and plan to provided written feedback on hardcopies.