In many recent studies of speeded scanning of immediate memory, variations in the size of the positive set (s) were confounded with variations in the probability (P) of the individual items within the positive set; as s increased, P decreased. The present experiment, with 48 undergraduates in an introductory psychology course, examined whether the effect on reaction time attributed to s could be accounted for by variations in P. This was accomplished by factorially varying both s and P. Probability effects were confined to items in the positive set and were insufficient to account for the effect of s. Results are discussed in terms of a model in which s and P affect different information-processing stages. The s affects the number of comparisons between the encoded item and the items stored in the memory of the positive set, as proposed by S. Sternberg (1966, 1967). The P affects response selection-information as to the particular digit that was presented was available to the mechanisms for response selection, along with the knowledge that there was or was not a match. The response selection mechanisms are assumed to be biased in tune with the P values of the items within the positive set. (18 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved)