Two component model of memory
The two component model of long-term memory asserts that two variables are sufficient to describe the status of unitary memory in a healthy human brain:
- stability (S) - this variable determines how long a memory can last if undisturbed and if not retrieved
- retrievability (R) - this variable determines the probability of retrieving a memory at any given time since the last review/recall
In the literature on memory, an ill-defined term of memory strength will often stand for either stability (S) or retrievability (R). This results in a great deal of confusion that slows down the progress of memory research. In 1990 and 1995, we have proven theoretically that the two variables: S and R are sufficient to describe the status of memory in spaced repetition.
The ultimate practical proof for the correctness of the two component model comes in the implementation and practical outcomes of Algorithm SM-17 in SuperMemo.
Independent research by Bjork, formulated as New Theory of Disuse (1992), uses analogous terms of storage strength and retrieval strength.