This model of memory suggests that the decay of a memory over time can be accurately described by two parameters: {retrievability} and {stability}.
Retrievability is defined as the {probability of a person remembering a fact after a period of time}. Stability is defined as {the durability of the memory: how long it will last if not retrieved}. Their relationship is approximated by {R ∝ exp(-t / S)}.
Stability increases with each review, so that retrievability falls more slowly with each repetition (see Spacing effect). Retrieval appears to increase stability more when retrievability is low: that is, Less accessible memories are more reinforced by retrieval (see also Desirable difficulties, after Bjork). Supermemo tries to characterize this relationship with an exponential curve, but the model complexity is high enough that I’m skeptical.
This model explains why it can be helpful to study even material which one can successfully recall (“overlearning”): even when retrieval strength is relatively high, study can increase storage strength, which will decrease subsequent retrieval strength decay (Robert A. Bjork and Bjork, 1992).
In practical use as part of a Spaced repetition memory system, a third parameter for item complexity must generally be introduced, since SRS items are generally not actually atomic, as far as our brains are concerned.
Piotr Wozniak uses retrievability and stability in his analysis; Robert A. Bjork and Bjork (1992) call an analogous pair of parameters retrieval strength and storage strength, respectively.
Q. Express two-component memory model’s stability component analytically.
A. Stability is the time interval within which retrievability is above some threshold, e.g. 90%.
Two component model of memory - supermemo.guru
Bjork, R. A., & Bjork, E. L. (1992). A new theory of disuse and an old theory of stimulus fluctuation. In A. F. Healy, S. M. Kosslyn, & R. M. Shiffrin (Eds.), Essays in honor of William K. Estes, Vol. 1. From learning theory to connectionist theory; Vol. 2. From learning processes to cognitive processes (p. 35–67). Lawrence Erlbaum Associates, Inc.