What is the spacing effect, and how strong is the evidence for it?

Distributing study across time beats cramming it — one of the most robust findings in learning science, backed by a meta-analysis of 254 studies.

The spacing effect says the same total study time produces more durable memory when it's spread out rather than massed. Cepeda, Pashler, and colleagues (2006, in Psychological Bulletin) meta-analysed 254 studies covering more than 14,000 participants and found spaced practice reliably and substantially outperformed massed practice. A useful rule of thumb from this literature: any gap beats no gap. (The finer question of the optimal gap is a topic for a later mission.)