As an illustration of odds of recall, we tried fitting an ACT-R model embodying this equation to the data of Runquist (1983) who looked at the probability of recalling paired associates at delays from 20 minutes to 21 days. The paired associates had either been studied 1 time or 3 times. To fit this data we estimated just the two parameters t and s with the decay rate d fixed at .5 and b = 0. The overall correlation is .93 and ACT-R is capturing the basic forgetting functions. The data itself appears a little noisy with some random non-monotonicities in the retention curves.