Graduated-interval recall is a shorter-timescale variant of spaced repetition, published by Paul Pimsleur in 1967.[1] It is used in the Pimsleur language learning system and it is particularly suited to programmed audio instruction due to the very short times (measured in seconds or minutes) between the first few repetitions, unlike other forms of spaced repetition which may not require such precise timings.
The intervals published in his paper were: 5 seconds, 25 seconds, 2 minutes, 10 minutes, 1 hour, 5 hours, 1 day, 5 days, 25 days, 4 months, 2 years.
Graduated Interval Recall is a complex name for a very simple theory about memory. No aspect of learning a foreign language is more important than memory, yet before Dr. Pimsleur's work, no one had explored more effective ways for building language memory.
In his research, Dr. Pimsleur discovered how long students remembered new information and at what intervals they needed to be reminded of it. If reminded too soon or too late, they failed to retain the information. This discovery enabled him to create a schedule of exactly when and how the information should be reintroduced.
By timing a Pimsleur language program with a stopwatch, it is possible to verify that the intervals are not followed exactly but have upper and lower bounds. A similar principle (graduated intervals with upper and lower bounds) is used in at least one open source software project (Gradint) to schedule its audio-only lessons.
http://www.sybervision.com/pimsleurphp/gir.htm