On testing the strength independence assumption in retrieval-induced forgetting

Strength independence refers to the assumption that in a retrieval-induced forgetting paradigm, the increase in performance for the practiced items (RP+) is independent of the decrease for the related and supposedly inhibited items (RP-). One way in which this assumption has been tested is by examining the correlation over subjects between these two measures. The finding that there is no such correlation has been taken as evidence for the inhibition account and against noninhibitory accounts of retrieval induced forgetting. We report several, large-scale simulation studies using a simplified version of the SAM model (Raaijmakers & Shiffrin, Psychological Review, 88, 93-134, 1981). The results clearly show that such a noninhibitory model is not likely to predict a significant correlation, despite the fact that on the level of the predicted probabilities such a correlation is clearly present. Additional simulations show that this is a very general result and not specifically related to the SAM model that was used. We conclude that such correlations do not provide a good test for the strength independence assumption and will not be able to distinguish between inhibitory and noninhibitory explanations of retrieval-induced forgetting.