Evidence file: knowledge-building curriculum

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
Grissmer et al. 2023(opens in a new tab)	Kindergarten lottery study	2,310 applicants to 9 Colorado Core Knowledge charters (UVA(opens in a new tab))	Lottery losers who attended other schools	About 16 percentile points higher in reading, grades 3–6; income gap closed at the one low-income school	Curriculum and charter model bundled
Kim et al. 2023 (MORE)(opens in a new tab)	Cluster randomized trial	30 schools, nearly 3,000 students, grades 1–2	Business as usual	Science reading comprehension up (0.18); smaller summer losses	Overall district reading score not significant in WWC review (WWC(opens in a new tab))
Tyner & Kabourek 2020(opens in a new tab)	Correlation, national longitudinal data	Elementary students, ECLS-K 2011	Less social studies time	More social studies time linked to better reading; more ELA time was not	Not causal
Recht & Leslie 1988(opens in a new tab)	Lab study	64 seventh and eighth graders	Students with less baseball knowledge	Baseball knowledge, more than reading ability, predicted comprehension of a baseball text	Small; see critique below

Counter-evidence and cautions

The baseball study is weaker than its fame. Mark Seidenberg points out that the "good" and "poor" reader groups did not actually differ much in reading skill, so the study cannot compare knowledge against reading ability (Seidenberg(opens in a new tab)). The site cites it as an illustration, not proof. The trials carry the weight.
Knowledge doesn't replace decoding. Both are necessary. Principle 9 handles decoding.