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The US Department of Education's What Works Clearinghouse (WWC) published an evidence review of World of Words preschool program to improve children's vocabulary/language. Quick take: WWC's finding of "strong evidence" is far overstated; the results are preliminary & not yet reliable.


  • World of Words aims to build children’s vocabulary, concept knowledge, & science content knowledge through topic-centered conversations & shared book readings.


Studies & Findings:

  • WWC’s review found "strong evidence" of a large impact on language outcomes (moving the average child from 50th to 66th percentile).

  • But WWC based its review on 4 studies that are only preliminary in nature: small samples, only short-term follow-up, & reliance primarily on researcher-designed (vs standardized) outcome measures - which can greatly inflate effect sizes.

  • Specifically, the 2 highest-quality studies (RCTs meeting WWC standards without reservations) each had samples of just 5-6 treatment classrooms vs 5-6 control classrooms, & lasted only 12-24 weeks. They found large impacts on researcher-designed tests, but no impact on established, standardized tests.

  • Such preliminary studies can be valuable in identifying programs warranting further research. But they can't by themselves produce reliable evidence, as their findings too often don't hold up in larger, more definitive RCTs.

  • And even large RCTs using standardized tests - such as HHS's Head Start impact study - have found that positive impacts in preschool too often don't lead to enduring effects on cognitive or literacy outcomes in early grade school.



  • In sum, World of Worlds may be a good candidate for further study, but WWC should make clear to education officials & educators that there's not yet reason to believe it meaningfully improves children's educational outcomes.

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