Spacing effect

You’ll remember material more reliably if you study it on many occasions, with some space in between—rather than if you spend the same amount of time cramming it all in one evening.

Your likelihood to forget some material follows an exponential decay curve, first characterized by Ebbinghaus in the mid-19th century. Reinforcing material immediately isn’t very useful because you’re unlikely to have forgotten it: Less accessible memories are more reinforced by retrieval. If you wait a while, you’re more likely to have forgotten it.

Each successive reinforcement appears to flatten that forgetting curve, so you can wait longer and longer between each review. The dynamics of that flattening effect are not well characterized (for one model see Two-component model of memory), but Spaced repetition memory system algorithms have used constant factors with relative success, which creates an exponentially-increasing series of test intervals, e.g. 5 days, 2 weeks, 1 month, 2 months, 4 months, etc.

Collected empirical evidence

Kornel (2009), in an experiment involving GRE-type vocabulary:

Combining the three experiments, 90% of participants learned more in the spaced conditions than the massed conditions, whereas only 6% of participants showed the reverse pattern.

Possible explanatory theories for the spacing effect

Drawn from Nate Kornell (2009):

  1. Spacing varies the context, which leads to richer, more diverse encodings.
  2. Spacing demands less concentrated effort and focus than massed study.
  3. Spacing reinforces memories when they are less accessible, which may enhance learning via Two-component model of memory


Branwen, G. (2009). Spaced Repetition for Efficient Learning. Retrieved December 16, 2019, from

Kornell, N. (2009). Optimising learning using flashcards: Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9), 1297–1317.

* On the Spacing effect:

  • [[Rohrer, D., Dedrick, R. F., Hartwig, M. K., & Cheung, C.-N. (2020). A randomized controlled trial of interleaved mathematics practice. Journal of Educational Psychology, 112(1), 40–52.]]
* Interleaved practice creates a natural context for the Spacing effect and Testing effect: