. Author manuscript; available in PMC: 2009 Nov 1.

Published in final edited form as: Psychol Rev. 2009 Jul;116(3):499–518. doi: 10.1037/a0016104

Table 1.

The model equations of the six retention models. In each equation, the symbol p (0 < p < 1) denotes the predicted probability of correct recall as a function of time interval t with model parameters a, b and c. Note that for the power model (POW), the decay factor is defined as (t+1) so that the probability remains between 0 and 1 for all positive t values

Model	Equation
Power (POW)	p = a(t+1)^-b
Exponential (EXP)	p = ae^-bt
Hyperbolic (HYP)	p = a/(a+t^b)
Power with asymptote (POWA)	p = a(t+1)^-b+ c
Exponential with asymptote (EXPA)	p = ae^-bt+ c
Exponential with exponent (EXPE)	p = ae^-bt^c