Moodle offers a number of different ways to calculate course grades. The St. Olaf Moodle instance allows you to choose from among four different types. These different aggregation types can be mixed and matched within a course, where the course aggregation type can be different than individual category aggregation types, which can differ from one another. Most courses will be wellserved by using Natural aggregation (the default for our Moodle) for the entire course. However, particular circumstances may call for the use of one of the other types. These aggregation types are summarized in the table below, and detailed with information below on how each one works, what it is useful for, and what limitations it has.
Aggregation types overview
Type

Based on

Drop lowest?

Scale Values

Extra credit?

Natural

Total points or
Avg. of weighted %

Yes (if all weighted same and same poss. points)

1, 2, 3, ...

Yes

Mean of grades

Avg. of %

Yes

0, 1, 2, ...

No

Weighted mean of grades

Weighted avg. of %

Yes

0, 1, 2, ...

No

Simple weighted mean of grades

Total points

Yes

0, 1, 2, ...

Yes

Natural (Σ)
Briefly:
Natural weighting automatically calculates a percentage weight for each item or category based on the points that item or category contributes to the total . Each calculated weight can be optionally overridden.
Useful for:
Natural weighting is the most versatile type, and can handle the majority of what you will want to do in your gradebook.
Features:

Weighting based on: Points or weighted percentage for each item

Extra credit possible: Yes

Drop lowest possible: Yes (in certain cases)

Extra credit and dropping cannot be used together

Reports category/course total in total points (Student user report also shows %)
Details:
By default, Moodle is set to use Natural weighting. As such, a 10point assignment has a default worth half of a 20point assignment. In dealing with categories, Moodle will look at the total points in that category compared to the total overall points to determine its default percentage (e.g. a category with 600 points in a course with 1000 total points would be worth 600/1000 or 60%).
For any item or category, you may choose to check the box next to the default calculated weight and enter an override between 0 and 100 (say you had an assignment that was worth 20 points, but you want it to be worth the same as the other 10point items in that category). If you manually set one value, Moodle will subtract that amount from 100 and redistribute the remaining percentage amongst the other items, according to their total possible points (e.g. assigning one of three 100point quizzes in a category to 20 [20%] will leave 80% to distribute to the remaining two, at 40% each, as they have the same total possible points).
This has been set as the default gradebook aggregation type as it does what most people expect (three 100point tests in a category are each worth 33% of a Quiz category), but also allows for adjustments within a category, and setting each category’s weight, regardless of the number of points in the category (for example, a Homework category can be set to 20% of a 1000point course, even though all of the assignments add up to 500 points, or 50% of the points).
Extra credit categories or grade items are possible with this aggregation type. You may also choose to drop the lowest X number of grade items in a category. However, all the items in that category must have the same number of possible points, or this option will be excluded. Additionally, scaled grades in this type are given numerical values starting at 1 and not 0, so that a Complete/Incomplete grade will be worth 1 or 2. If you wish to use your scaled grades in calculations with the lowest value equal to zero, you'll need to choose another type. In either of the previous cases, you will most likely want to set the category to use Simple weighted mean of grades instead.
Mean of grades (x̄)
Briefly:
Mean of grades calculates every grade item as a percentage and computes the average of all items in a category for its total, or the average of all subcategories in a category or course.
Useful for:
Mean of grades is best for when you have multiple items with different point totals in a category, and you don’t want to manually set them to have equal weight.
Features:

Weighting based on: Percentage for each item

Extra credit possible: No

Drop lowest possible: Yes

Reports category/course total out of 100
Details:
Using Mean of grades means that there could be multiple exams in a test category, each with a differing point total, and each would have the same weight within the Tests category (for example, 3 tests of 90, 110 and 140 points would each be worth ⅓ of the Tests grade, with the three percentage scores added up and divided by 3). This is the same as using the Natural aggregation type and manually setting each test to 33.33%.
This aggregation type could be useful for a category with multiple items with different point values that should be treated equally based only on the percentage score of the item. This would eliminate the need to manually set each of the items to the same weighted value that natural weighting would require.
Mean of grades does not allow for extra credit items or categories, but does allow for the dropping of the lowest X grades in a category. In order to use extra credit, it is advisable to use Natural grading aggregation and setting the weights manually. If you wish to have extra credit and drop the lowest grade(s) in a category, then Simple weighted mean of grades with manually set weights is advised.
Weighted mean of grades (x̄)
Briefly:
Weighted mean of grades calculates every grade item and category as a percentage and initially assigns each item or category an equal weight of 1, meaning they are counted once in the average. Each item or category can be manually assigned a lower or higher weight to be counted more or less in the average (i.e. 0.5 for half as much, 2 for twice as much).
Useful for:
Weighted mean of grades is helpful when you want to assign relative weights to items in a category, and you don’t know what the total number of assignments will be.
Features:

Weighting based on: Percentage for each item (counted x times)

Extra credit possible: No

Drop lowest possible: Yes

Reports category/course total out of 100
Details:
Using Weighted mean of grades means that each item initially carries equal weight in the category, regardless of the number of total possible points available. Without manually changing weights, this is the same as Mean of grades. However, by adjusting the default weight of 1 for a grade item or category, you can increase or decrease the contribution to the total.
Simple weighted mean of grades (x̄) (=Natural without any weights)
Briefly:
Simple weighted mean of grades is the same as Natural without any weight overrides set. It simply adds up the points of each item in the category/course and divides by the total points possible. Like Natural, it can also accommodate extra credit items, with the points earned contributing to the total points earned, but the maximum points available does not add to the total possible.
Features:

Weighting based on: Point total for each item

Extra credit possible: Yes

Drop lowest possible: Yes

Extra credit and dropping can be used together

Calculates scales on a 0 > X scale, not 1 > X

Reports category/course total out of 100
Useful for:
The best use case for Simple weighted mean of grades is for when you want to use absolute point values, but need to drop the lowest or keep the highest grade and have items with different point totals, or if you don’t need weighting and you want to use extra credit and dropping the lowest X scores together. Additionally, this aggregation scheme assigns values to grading scales starting at 0, while others start at 1, so if you wish to do calculations with scaled grades, and want the lowest item to count for 0 points, you'll need to use Simple weighted mean of grades.
Details:
With Simple weighted mean of grades, the scores of all items are summed up and divided by the sum of all the possible points, exactly like in Natural weighting with no manual weight overrides.
Dropping lowest score(s)
Dropping capability
As noted in the summaries above, all of the aggregation types allow you to drop the lowest X score(s) in the category. There are two exceptions to this in the Natural aggregation type. The first is that dropping can only be used with Natural when all grade items in the category have the same total possible point values. The other is that dropping can only be used in Natural aggregation when no extra credit items exist. In both of these cases, if no weighting override is used, the alternative is to use Simple weighted mean of grades, which allows both situations above.
In the case where you have grade items with differing maximum point totals which need to be weighted equally (or simply have manual overrides) and you wish to use dropping, then the alternative is to use Weighted mean of grades. In order to also use extra credit in this situation, you must put the differing point total items into a category using Weighted mean of grades and put that category into another category using Natural or Weighted mean of grades with the extra credit item(s).
Dropping calculation
When determining which grade item to drop, Moodle will drop the lowest score by percentage, dropping the first item it encounters when 2 or more have equal percentage. In the case of Simple weighted mean of grades and Weighted mean of grades, the dropped grade will affect the overall category grade to a magnitude relative to the proportion of the total points or weight it represents.
This automatic determination could theoretically be beneficial to one student (dropping a high value low grade) and detrimental to another (dropping a low value low grade). In Simple weighted mean of grades, this occurs because grade items with a greater maximum point total represent a larger percentage of the overall grade, and in Weighted mean of grades, a grade item weighted greater than others (i.e. with a weight of 2 or 3, versus others with a weight of one) is effectively removing two or three grade items, whereas removing an item with a weight of 1 is removing only one low score. For this reason, it is recommended that dropping is only done where all items carry the same weight, or that some manual oversight is used when using it with unequally weighted items.
Extra credit
Both individual grade items as well as entire categories can be designated as extra credit, as long as they are in a category that allows for it. The easiest way to add course extra credit is to have the course set to Natural aggregation and have a courselevel (i.e. toplevel) grade item marked as extra credit, and set the weight of the extra credit item to be equal to the maximum number of points available, e.g. 5 maximum extra credit points possible, and 5.0 as the weight. As long as these two numbers are the same, the number entered into the gradebook will add that many extra credit percentage points to the total course grade.
When using Simple weighted mean of grades, it is much more difficult to set up an item that corresponds to specific extra credit points. For example, if a category has 500 total possible points from all grade items, an extra credit item would need to have a total maximum possible points of the following formula: # of possible extra credit points * 1% * total possible course points. For our example of 500 with 5 extra credit percentage points available, we would have 5 * .01 * 500 = 25. This would mean that there would have to be 25 points possible, and each extra percentage point would be entered as 25 / 5 = 5.
In cases where there is an extra credit assignment, which has similar total possible points to others in the same category, using either Natural or Simple weighted mean of grades aggregation could yield the desired result, adding the same weight as one of the grade items in that category.