A representative experiment which captures these results is Experiment 4 reported by Ratcliff, Clark, and Shiffrin (1990). In five conditions they had subjects either study 10 items 4 times (the 10-PS condition for "pure strong"), 16 items 4 times (the 16-PS condition), 16 items with half presented 4 times and half presented 1 time (the 16-M condition for "mixed" condition which contains 8 strong items, designated 16MS, and 8 weak items, designated 16MW), 16 items each presented once (the 16-PW condition for "pure weak"), or 40 items each presented once (the 40-PW condition).
Recently, a considerable stir has been caused by what are called the list strength and list length effects on recognition memory (e.g., Ratcliff, Clark, & Shiffrin, 1990). Recognition memory for individual items deteriorates as the list has more items (the list-length effect). It also gets better as items are studied more (the list-strength effect). The interesting question is what happens when some items of the list are studied more often or longer and others are not. By analogy to the list-length effect, one might imagine that if some items are studied more (this is analogous to making the list longer) the remaining items in a list would suffer greater interference. Just these effects occur with mixed lists in free recall where extra study for some items makes other items less available. However, in recognition memory there is no effect of amount of study of other items. Ratcliff, Clark, and Shiffrin (1990) proclaimed that no extant theory of memory could accommodate these results. Since that time a number of theories (e.g., McClelland & Chappel, 1994; Shiffrin & Steyvers, 1997) have been modified or proposed to accommodate the result. It turns out that ACT-R is in this list of theories although we have to say this result was far from our mind when we proposed the ACT-R theory in 1993 and we have only recently realized that it explained these effects.
A representative experiment, which captures these results, is Experiment 4 reported by Ratcliff, Clark, and Shiffrin (1990). In five conditions they had subjects either study 10 items 4 times (the 10PS condition for "pure strong"), 16 items 4 times (the 16PS condition), 16 items with half presented 4 times and half presented 1 time (the mixed condition which contains 8 strong items, designated 16MS, and 8 weak items, designated 16MW), 16 items each presented once (the 16PW condition for "pure weak"), or 40 items each presented once (the 40PW condition).