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. 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.


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