[ARCHIVED] Question about Methodology for Curving Grades in Canvas

[brennem1]

Hi,

I was studying the method Canvas uses to curve grades ("Curving Grades in Canvas PDF" ). I have a question about how the widths of the bin intervals (into which the scores are distributed) is done.

My understanding is that you define a minimum score (as 2*Xbar-N, where N = # pts on quiz) and a maximum score (N), then divide this up into 21 evenly spaced bins (with 22 "break points", which I believe are just the endpoints of the bin intervals)

But when you look at what I believe is the method for computing the bin widths in the step, "Calculating Final Scores" in step 2 it says "Divide that total by the remaining break points". By "total" I assume they mean the output from step 1, which is the difference between the max and min scores. But what does "the remaining break points" mean?

In the calculations you have 21 bins or 22 break points. In the sample calculation shown, the difference between the max and min scores is 22, but the division for step 2 is performed with 19, not 22. 

So a second (related) question is, how was the 19 (which I assume was the "remaining (number of ?) break points") computed?

Thank you,

Matt