Spaced repetition is a learning technique in which increasing intervals of time are used between subsequent reviews. Alternative names include expanding rehearsal, graduated intervals, repetition spacing, repetition scheduling, spaced retrieval and expanded retrieval.
More recently, spaced repetition has also been implemented into computer-assisted language learning software to gradually adapt the optimum spacing of repetitions to individual needs. A typical optimization criterion used in spaced repetition is the requested level of knowledge retention, i.e. percent of knowledge that is to be remembered.
There are several families of algorithms for scheduling spaced repetition:
The precise length of intervals does not have a great impact on algorithm effectiveness, as reflected in data collected by Mnemosyne.