The spacing effect is the finding that information is better retained when study is distributed across multiple sessions separated by intervals (spaced or distributed practice) than when the same total study time is concentrated in a single session (massed practice or cramming). First documented by Ebbinghaus in 1885 and replicated in thousands of studies since, the spacing effect is one of the most robust and replicable findings in all of learning science — and one of the most underutilized in educational practice.
The Phenomenon
The spacing effect has been demonstrated for virtually every type of material (words, facts, concepts, motor skills, problem-solving procedures), every type of learner (children through elderly adults, with and without learning disabilities), every retention interval (hours to years), and across laboratory and educational settings. Spacing can improve long-term retention by 10-30% or more compared to massed practice with identical total study time.
Test in 1 week → space reviews ~1-2 days apart
Test in 1 month → space reviews ~1 week apart
Test in 1 year → space reviews ~1 month apart
Explanations
Multiple mechanisms likely contribute. The deficient processing hypothesis proposes that massed repetitions receive less attention (because the material feels familiar) while spaced repetitions receive fuller processing (because partial forgetting requires re-encoding). The study-phase retrieval hypothesis proposes that spaced repetitions require retrieving the earlier presentation from memory, providing retrieval practice benefits. The contextual variability hypothesis proposes that different study sessions encode different contextual information, creating more diverse retrieval cues.
The spacing effect has been operationalized in spaced repetition software (SRS) such as Anki, SuperMemo, and Quizlet. These systems use algorithms to schedule reviews at expanding intervals, presenting items just before they would be forgotten (based on the user's performance history). SRS has been particularly effective for vocabulary learning, medical education, and any domain requiring long-term retention of discrete facts. Pimsleur's graduated interval recall method for language learning was an early application of the spacing principle.
The Spacing-Testing Interaction
Spacing is even more effective when combined with retrieval practice (testing). The combination of spaced retrieval practice — testing oneself on material at expanding intervals — is one of the most potent learning strategies available. This synergy between spacing and testing forms the basis of effective study strategies recommended by learning scientists and implemented in adaptive learning technologies.