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Item difficulty is a term used in SuperMemo to refer to the degree of effort needed to retain knowledge in memory. Difficult items are simply hard to remember.

Historically, item difficulty was reflected using concepts that kept evolving with the SuperMemo algorithm:

  • in SuperMemo on paper, all pages were assumed to have the same difficulty, however, difficult items could be rewritten to newly created pages. In addition, hard material could be listed on pages that would use a denser review schedule
  • in SuperMemo 1.0 through 7.0, difficulty was measured by E-Factors that may depend on the contents of the collection
  • in SuperMemo 8.0, difficulty was measured by A-Factors that made the first absolute measure of difficulty
  • in Algorithm SM-17, difficulty is obtained by the best fit of repetition history to the algorithmic predictions derived from the recall matrix
  • in Algorithm SM-18, difficulty is modified during repetition in proportion to missed expectations (see: Item difficulty in Algorithm SM-18)

Items that exceed the acceptable degree of difficulty are called "leeches". Usually, leeches violate the 20 rules of formulating knowledge and should be deleted or reformulated.

In the two component model of long-term memory, the term memory complexity is used instead of difficulty. Complexity refers to the neural structure involved in representing a memory. Difficulty depends on complexity, and other factors (e.g. interference, mnemonic skills, negative associations, etc.). In spaced repetition, difficulty may change without a change to the underlying complexity.

Item difficulty reflects memory complexity, but may change without a change to complexity

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