List Strength Effect Tutorial

Simon Dennis

An issue that has been of interest for more than a century is the affect on memory of study lists that contain items of different strengths (Ebbinghaus, 1964; Thorndike, 1965; Tulving & Hastie, 1972; Ratcliff et al., 1990). The strength of an item is manipulated either by increasing the number of times it occurs in the study list or by increasing the length of time for which it is studied. Items of greater strength are generally retrieved more accurately than weak items (Ebbinghaus, 1964; Thorndike, 1965). Of more recent interest is what happens to performance on the weak items as a consequence of strengthening other items within the list. Table 1 outlines the pattern of accuracy results for the weak items within different list types for recognition, cued recall and free recall.

Table 1: Accuracy results for nonrepeated items in different list types.

Test Type	AB vs ABB	AB vs ABC	ABB vs ABC
Recognition	= (>)*	>	>*
Cued Recall	= (<)*	>	>*
Free Recall	>	>	<

Note. Asterisks indicate the results that were not predicted by the mathematical memory models. = (<) means equal or perhaps slightly worse. = (>) means equal or perhaps slightly better. AB is a short list without repeated items. ABB is a long list with repeated items. ABC is a long list without repeated items. This table is compiled from results by Tulving & Hastie (1972) and Ratcliff, Clark & Shiffrin (1990).

For free recall, Tulving and Hastie (1972) demonstrated that performance on unstrengthened items dropped significantly in comparison to the AB and ABC control conditions. For recognition, however, strengthening an item does not degrade performance significantly on the non-strengthened items (known as the null list strength effect, Shiffrin et al., 1990; Ratcliff, 1990; Yonelinas et al., 1992). This result contradicted the major mathematical memory models all of which predicted that performance on A in the ABB condition would be worse than that for A in the AB condition and should be as poor if not worse than that in the ABC condition (Ratcliff et al., 1990).

Measuring List Strength

The analysis of the list strength effect has included the calculation of a ratio of ratios measure given by the following formula (Shiffrin et al., 1990; Ratcliff et al., 1990):

Rr = d'(mixed strong)/d'(mixed weak)
     -------------------------------
     d'(pure strong)/d'(pure weak)

If weak items are not impaired by strengthening other items in the list (and conversely, if strong items do not improve by weakening other items in the list), the ratio of ratios measure will be near one. Ratio of ratios near or somewhat below one have been found in several recognition studies (Ratcliff et al., 1990).