Evidence file: quizzing from memory

Study label	Meaning
Randomized trial	Students, classes, or schools assigned by chance. The strongest single-study design.
Lottery study	Admission by lottery, which works like random assignment for those who applied.
Meta-analysis	A combined analysis of many studies; only as strong as the studies inside it.
Comparison study	Groups compared without random assignment. Useful, but differences may come from who chose what.
Lab study	Controlled experiment outside a regular classroom. Explains why; doesn't prove it works at scale.
Correlation	Two things move together. Shows a link, never cause. Used only for "why it matters."

Study label

Meaning

Randomized trial

Students, classes, or schools assigned by chance. The strongest single-study design.

Lottery study

Admission by lottery, which works like random assignment for those who applied.

Meta-analysis

A combined analysis of many studies; only as strong as the studies inside it.

Comparison study

Groups compared without random assignment. Useful, but differences may come from who chose what.

Lab study

Controlled experiment outside a regular classroom. Explains why; doesn't prove it works at scale.

Correlation

Two things move together. Shows a link, never cause. Used only for "why it matters."

Study	Design	Who	Compared with	Found	Limits
Yang et al. 2021(opens in a new tab)	Meta-analysis of classroom studies	222 studies, about 48,000 students	Other learning activities or none	Quizzing raised achievement by about half a standard deviation	Mixed designs and comparison groups
Agarwal, Nunes & Blunt 2021(opens in a new tab)	Systematic review of classroom experiments	50 experiments, 5,374 students, K–12 to medical school	Business-as-usual teaching	57% of effects were medium or large	Benefits smaller when final test questions were reworded
McDaniel et al. 2011(opens in a new tab)	Classroom experiment	8th grade science	Content not quizzed	13–25% gains on unit exams; benefits lasted to semester and end-of-year exams	One school
Roediger et al. 2011(opens in a new tab)	Classroom experiments	6th grade social studies	Not quizzed, or content shown twice	Better chapter and semester exams after quizzing	One school
Roediger & Karpicke 2006(opens in a new tab)	Lab experiments	College students	Restudying	Restudying won at 5 minutes; testing won at 2 days and 1 week	Lab, adults, short passages

Counter-evidence and cautions

The lab finding that students feel more confident after rereading, even though they remember less, means students and teachers will underrate quizzing (Roediger & Karpicke 2006(opens in a new tab)).
Transfer to reworded questions is real but smaller. Quiz on ideas, not only exact wording.
Most classroom studies were run in the U.S. and Western Europe (RetrievalPractice.org(opens in a new tab)).