Every real mark a student has typed in from what a teacher handed back, next to how much they
practised on here in the same weeks. The window is split in half; the change is second half minus first.
Practice is rounds of practice questions and testing; checks, help and essay reads are counted beside it.
This is not proof. Marks are self-typed, the window is short, and a pattern across a few dozen
students is a hint. The number of students with marks in both halves is shown because a mean over three
people is not a mean.