How strong is the evidence for interleaving? Name the meta-analysis and its effect size.

Moderate overall, strong for discrimination — Brunmair and Richter (2019) meta-analyzed 59 studies and found an overall Hedges' g = 0.42.

Brunmair and Richter, publishing in Psychological Bulletin, pooled 59 studies of interleaving versus blocking. Hedges' g is a standardized effect size (like Cohen's d, but corrected for small samples); g = 0.42 is a solid, meaningful benefit but not enormous. The headline finding was that similarity matters: interleaving helped most when the categories were easy to confuse — for distinguishing painters' styles the effect reached g = 0.67, while for mathematics it was smaller at g = 0.34. This is well-replicated but nuanced science, not a universal miracle.