The list-strength effect: Strength-dependent competition or suppression?

If several items are associated with a common cue, the cued recall of an item is often supposed to decrease as a function of the increase in strength of its competitors' associations with the cue. Evidence for such a list-strength effect has been found in prior research, but this effect could have been caused both by the strength manipulations and by retrieval-based suppression, because the strengthening and the output order of the items were confounded. The experiment reported here employed categorizable item lists; some categories in each list contained strong items only, some contained weak items only, and some contained both strong and weak items. Strengthening was accomplished by varying the exposure time of the items. The testing sequence of the items from each category was controlled by the use of category-plus-first-letter cues. When the typical confounding of strengthening and output order was mimicked, list-strength effects were found, which is consistent with prior research. However, when this confounding was eliminated, the list-strength effects disappeared: The recall of neither strong nor weak items varied with the strengths of the other category exemplars. This pattern of results indicates that the list-strength effect is not the result of strength-dependent competition, but is caused by output-order biases and a process of suppression.