This is exciting because the effect size is so large. But as the author's acknowledged, selection bias is nearly impossible to control for in this non-randomized study:

> and lacks randomized controls. Self-selection is the central threat: students who complete more quizzes may be more motivated or higher-performing generally

But this is still a strong result. I'm excited to see more in this space.

They tried to control for this. It's described in the first paragraph of section 4.

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