The Sternberg paradigm is one in which subjects see a relatively small list of items and then are presented with a single item and have to judge whether that item is from the list. As the result was originally described and is still described in many textbooks, the claim is that there is a linear relationship between the number of items in the memory set and time to make this judgment. In fact the relationship is more typically curvilinear and extends out to lists as long as 20 items in length (Briggs, 1974).
To have better tests of ACT-R it would be useful to have data with better specification of the presentation timing and information about the effect of serial position on latency. Raeburn (1974) reports such an experiment in which items were presented at the rate of 1.5 seconds per item and tested at a delay of 1.2 seconds.