Note on guessing probabilities

A hypothesis of stationary response probabilities (SRP), or that subjects equate the unconditional probability of each response to its frequency of occurrence in the task, is shown to be equivalent to the proposal of Atkinson et al. (1965) that guessing may occur only among unlearned responses. This hypothesis is contrasted with the usual assumption of stationary guessing probabilities (SGP). It is shown that assuming SGP when SRP is true results in a biased estimate of learning which first increases and then decreases with the actual degree of learning, and is a decreasing function of list length. This can lead to improper inferences, including erroneous rejection of an all-or-none model and spurious evidence of increasing difficulty in long lists.