Initially characterized by Hermann Ebbinghaus, the “forgetting curve” describes a class of related power-law phenomena around forgetting and memory: roughly speaking, memory “retrieval strength” (see Two-component model of memory) seems to decrease along a power-law curve after initial exposure.
Ebbinghaus’s initial experiments established the forgetting curve in terms of the amount of effort required to re-learn nonsense syllables after some time had passed. More time was required as the session interval increased, along what he believed to be an exponential curve.
Subsequent literature (e.g. Wickelgren, 1974) reframed this effect as a probabilistic one (and a power-law rather than exponential): probability of recall falls over time along an inverse power-law curve, modulated also by an exponential component accounting for interference.
Wixted and Carpenter (2007) summarize the forgetting curve as:
P(recall) = m(1 + ht)^-f
Where m is the degree of initial learning (i.e. the probability at time 0), h is a scaling factor on time, and f is the exponential memory decay factor.
Wickelgren, W. A. (1974). Single-trace fragility theory of memory dynamics. Memory & Cognition, 2(4), 775–780.
Wixted, J. T., & Carpenter, S. K. (2007). The Wickelgren power law and the Ebbinghaus savings function. Psychological Science, 18(2), 133.