This article explains weighted grades and why some professors use them.
What does it mean to weight a grade? When grades are weighted, some assignments (e.g. Final Essay) or categories of assignments (e.g. Lab Experiments) count more than others.
Why not just assign more points to the things that are harder? In a perfect world this would work. However, there is far more flexibility in weighting grades. If a professor decides that quizzes should make up 25% of the total points in a course, it doesn't matter whether there is 1 quiz, 5 quizzes or, heaven forbid, 50 quizzes. The total points earned for the quizzes still makes up 25% of the the final grade.
If a professor decides to increase or decrease the workload based on the particular needs of any group of students, weighted grades make that easy. In a scenario where grades are weighted by category and the number of assignments in a category changes, no changes to the course points or syllabus would be necessary. If, on the other hand, the point system were used, and a change to the course assignments is needed, the total points for the class would change and the syllabus would need to be updated.
How are the points for a weighted item calculated? Mutliply each score over the total points possible by the weight. For example, the midterm is worth 100 points. If you earned 90/100 the calculation would be .25(90/100)=.225. Expressed as a percent, you earned 22.5%
Blackboard calculates weighted grades both proportionally and equally. What's the difference? Equally vs. Proportionally is a setting when you weight by category. It only makes a difference if you have columns with different points possible in the same category (e.g. a Quiz category with a column worth 10 points and column worth 20 points). If all of the columns in the category are equal, both settings work the same way.
When categories have different values, equal weighting converts the columns to percentages and averages the percentages to get the category total grade. It essentially gives each item equal weight when determining the total grade. Proportional weighting calculates a category total grade by adding the raw scores and dividing by the total points possible. It maintains the proportional weight of each item, so items with a larger value have more effect on the total grade.
For example, consider two assignments in a category, one worth 10 points and the other worth 20 points. Assume the student gets 10 points on each assignment.
Equal weighting: 10/10 and 10/20 = (100% + 50%) / 2 = 75% (or you can think of converting it to equal points possible: 20/20 and 10/20 = 30/40)
Proportional weighting: 10/10 and 10/20 = 20/30 = 66.7%
The category total grade is then weighted according to the percentage you indicated for the entire category and combined with the other columns or categories you have included in the Weighted Total.
Sample weighted grade book with formulas and VLOOKUP for letter grade. (NOTE: You must have Excel installed on your computer to view this file. There are multiple ways to compute weighted grades. This spreadsheet demonstrates one method.)