What a wrong answer actually tells you

A score tells you how many. It does not tell you which idea a class turned the wrong way, and that is the only part you can teach to.

Two students can both score six out of ten and need completely different lessons. One has the method and drops signs; the other has no method at all.

Written answers are where this shows up. The marking here scores against an answer you wrote yourself, and shows the reason — so you read the thinking, not the tally.

Then you teach the idea that failed, and check it again next week.