What did Rohrer, Dedrick, and Stershic (2015) show about interleaving and forgetting?

7th-graders scored 80% vs 64% (interleaved vs blocked) at 1 day, and 74% vs 42% at 30 days — near-immunity to forgetting.

This study is striking because of what happens over time. At a one-day delay the interleaving advantage was real but modest (80% vs 64%). By 30 days the blocked group had collapsed to 42% while the interleaved group barely moved, holding 74%. In other words, interleaving didn't just raise the starting score — it made the learning far more resistant to forgetting. The desirable difficulty paid off most exactly when it mattered: weeks later, when a normally-studied skill would have faded.