Forgetting index

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Forgetting index is a term used in SuperMemo to name the acceptable degree of forgetting (expressed in percent). For example, if the student agrees that recall of 90% is acceptable, he can set the forgetting index to 10% in SuperMemo. Reducing the forgetting index does not pay as it exponentially increases the cost of learning. Increasing the forgetting index is equivalent to giving up on the benefits of spaced repetition. SuperMemo allows of the forgetting index in the range from 3% to 20%.

The best solution to the retention-workload dilemma is to employ the priority queue in incremental reading where only top-priority knowledge is subject to strict review (as determined by the forgetting index). In addition, incremental reading capitalizes on the concept of knowledge darwinism that makes it easy to establish low-cost high-value memories.


  • requested forgetting index: the desired and planned value of the forgetting index
  • measured forgetting index: the actually achieved value of the forgetting index
  • individual forgetting index: the value of the requested forgetting index assigned to an item
  • default forgetting index: default value of the individual forgetting index assigned to items added to a collection
  • expected and estimated values of the forgetting index are terms used only in SuperMemos 8 thru 16 (see: Algorithm SM-15)

For more see: Forgetting index in SuperMemo

This glossary entry is used to explain SuperMemo, a pioneer of spaced repetition software since 1987