Hello @bkohn 
Thank You for your question. The nice thing about Canvas is that you don't have to tell Canvas to do this for you. It's already computed in the SpeedGrader. (I just learned about this a week or two ago.) When you go to the SpeedGrader for your test, you'll see your student names (in a drop-down list) at the top right corner of the screen (just like you would with any graded assignment). To the immediate left of the student name area it will show how many tests have been scored so far (the below example from my sandbox course shows that 3 out of 7 quizzes have been graded...the fictional scores were 0, 10, and 6 ... out of 10 possible points). Then, it gives the average of those scores...in this case, 16 (combined total points of people who took the quiz) ÷ 3 (number of people who took the quiz) = 5.33. Hope this helps, Brian.
