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-randomize

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.