Extra Credit Items are especially tricky in the Moodle gradebook, because their value can vary greatly depending upon where they are placed, what aggregation method is used, and the maximum point setting for the course total.
NOTE: The Extra Credit option is not available when you are creating a new grade item. You must create and save the item and then edit it later to set the Extra Credit option.
If there are no sub-categories in the gradebook setup, the extra credit item's value will be added to the course total and calculated according to the aggregation method of the overall course category.
If an extra credit item is placed within a sub-category in the gradebook setup, its value will be calculated within that category first, depending on the aggregation method for that category. The total of the sub-category will then be used in the calculation of the parent categories, according to their aggregation method.
Some aggregation methods (such as Simple Weighted Mean of Grades) will automatically set the course total to 100, no matter what the sum of all grade items. In that case the extra credit item points will count as extra percentage points (on a 100 point scale).
If you use the aggregation "Natural" method, the course total will automatically adjust to the sum of all credit items. In that case the point of the extra credit items will be added to the sub-category or course total points, which will be divided by the total points possible. For example if the total points possible is 500, then each extra credit point will equal .2 % of a final percentage point.
To add an extra credit item: