I have never used increments but here is what is happening - you say .5 increments start at 15 - so a grade of 21 if each point after 15 equals .5 brings an actual total of 18 which is exactly 60%.

Well, the math works at least!

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I have never used increments but here is what is happening - you say .5 increments start at 15 - so a grade of 21 if each point after 15 equals .5 brings an actual total of 18 which is exactly 60%.

Well, the math works at least!

Thank you, Emma! That does actually solve the mystery and it highlights for me the perils of using scales because it appears they don't work at all the way one would intuitively expect. In the example above, it would appear the actual SWM calculation that Moodle is using is based on the number of increments rather than the point values associated with those increments. Here's what we came up with (my more math-literate colleague helped me get to this):

(0.3*21+0.6*46+1*21+0.8*21+0.7*21+0.867*31+0.9*21+0.8*21)/203=73.3%

So, the math checks out in the end. But I can't for the life of me see the value in scales that don't have uniform increments (there are several in the standard scales set. Indeed, the only value I can see to any numerical-based scale is in the case of needing to create an activity with points greater than 100 (a separate conundrum). Is there a way I can disable these scales? IF I did disable them, does anyone know how that would affect grades based on those scales?