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To test the idea that unannounced quizzes improved performance on later hour exams, I collected test scores from 4 classes over 2 semesters. I compared course segments with quizzes to segments without quizzes in a counterbalanced design. Test scores following quizzes averaged half a letter grade higher than scores from segments without quizzes. Mid-range students benefitted the most, with C students gaining 84% of a letter grade. Attitude surveys revealed a high level of student acceptance for the quizzes.
Robert B. Graham (1999) studied this question.