Spacing effect
Learning spread out over time produces far better retention than cramming.
What it means
The spacing effect is the finding that information is retained much better when study or practice is distributed across time than when it is massed into a single concentrated session. The leading mechanism is that each retrieval attempt made after some forgetting has set in is more effortful, and that desirable difficulty strengthens the memory trace more than easy, immediate repetition; complementary accounts point to encoding variability (spaced sessions occur in different contexts, creating more retrieval cues) and to reduced reliance on fluent, short-lived familiarity. Cramming feels productive because it yields high performance in the moment, but that fluency is misleading and fades quickly, whereas spaced practice trades a little short-term ease for large long-term gains. The effect is one of the most robust and replicable findings in the science of learning, holding across ages, materials, and species, and it interacts with the testing effect, since spaced retrieval practice is especially powerful. It matters because it directly contradicts the most common study habit, implying that schools, training programs, and self-learners should distribute and interleave practice rather than block it.
Examples
Reviewing vocabulary on days 1, 3, and 7 produces far better retention than studying it three times in a single evening.
Twenty minutes of guitar on six evenings builds far more than one two-hour Sunday session, even though the long session feels far more productive while you are doing it.
Fire drills run once a quarter keep the evacuation route available for years; the same minutes spent inside a single induction morning are mostly gone by the summer.
First described in Hermann Ebbinghaus (1885).