When pairs of letters or letter strings are classified as "same" or "different," correct "same" responses often are faster than correct "different" responses. Researchers have disagreed about the importance of this fast-same phenomenon, with some regarding it as indicative of a critical, basic characteristic of pattern recognition processes and others regarding it as a byproduct of response criteria. The present experiments examined this issue for multiletter matching. Two procedures were used, one in which response bias was manipulated by varying the proportion of same pairs and another in which string length effects were examined for a situation designed to minimize premature termination of the comparison process. The procedures provided converging results, indicating that, although the fast-same effect customarily obtained with simultaneous presentation of the pair of letter strings is likely due to response bias, the effect obtained with successive presentation is not. With successive presentation, same strings have a processing advantage over different strings that apparently is due, at least in part, to facilitation in the encoding of the second string.In pattern-matching tasks, subjects classify pairs of stimuli as either "same" or "different." When the stimuli within the pairs are single letters, multiletter strings, or multidimensional nonalphanumeric stimuli, correct "same" responses often are faster than correct "different" responses (Krueger, 1978;Nickerson, 1978). The fast-same phenomenon has been the subject of intensive investigation because it is inconsistent with most feature-analytic models of the comparison process. These models predict that, if anything, "different" responses should be faster than "same" responses, because a single difference in component features is sufficient to indicate "different," whereas all features must match to indicate "same" (Bamber, 1969; Nickerson, 1965). The violation of this prediction of feature-analytic models suggests that the fast-same phenomenon may provide a fundamental insight into the processes that underlie matching-task performance.Because of its apparent importance, the fast-same phenomenon has been the focus of several patternmatching models (Bamber, 1969;C. W. Eriksen, O'Hara, & B. A. Eriksen, 1982;Krueger, 1978;Proctor, 1981;Taylor, 1976). One exception is the model developed by Ratcliff (1981), which dealt only cursorily with the fast-same phenomenon, because This research was supported in part by a research Grant-in-Aid from Auburn University. We would like to thank Dennis Hanks, Stephanie Ford, Mitch Jones, Jim Parker, and Hunter Peak for assistance in the collection and scoring of the data. We would also like to expressour gratitude to T. Gilmour Reeveand Peter Harzem for allowing us use of their computer facilities and to James McAlarney III for programming the experiments. Requests for reprints should be sent to Robert W. Proctor, Department of Psychology, Auburn University, Auburn, AL 36849. the phenomenon was regarded as "little more thana...

Proctor (1981) presented a theoretical framework that distinguishes factors contributing to disparities in time to classify physical-same, name-same, and different letter pairs as a function of three variables: method of presentation (simultaneous vs. successive), case relationship (samecase vs. opposite-case pairs), and blocking (blocked vs. mixed presentation of same-case and opposite-case pairs). He also argued that these variables were critical in multiletter matches, and performed a between-study comparison of existing data to support his contention. Because comparison across studies is always a tenuous process and because the absence of several relevant conditions precluded a complete analysis of predictions, a within-experiment manipulation of the three relevant variables was desirable. The present study reports such an experiment. In general, the factorial manipulation of variables supported predictions of Proctor's framework and indicated that many phenomena of multiletter matching, including the widely studied fast-"same" phenomenon, are attributable primarily to differences in the rate at which component letter pairs are matched.Numerous studies during the past decade have examined same-different matches of multiletter strings. Such studies have been popular not only because they allow precise quantitative tests of the manner in which multielement stimuli are compared (e.g., Bamber, 1969; Taylor, 1976b), but also because they provide a baseline for examining the influence that various structural constraints (such as orthographic regularity, phonological regularity, and word structure) have on processing (e.g., Bruder, 1978;Bruder & Silverman, 1974).In the most common version of the multilettermatching task, two letter strings are presented that are of the same length, and subjects are required to respond "same" if all letters in corresponding positions are identical (e.g., MDT B; MDT B) and "different" otherwise. The general pattern of results obtained in all studies of this type is that the latencies of both "same" and "different" responses increase as a function of string length and that, within a given string length, the latency of "different" responses increases with the number of positions on which the two strings match.In the first study of multiletter matching, Bamber (1969) determined that the "different" reaction-time functions were accurately described by an analyticalWe would like to thank Peter Harzem for allowing us to conduct the experiment on his equipment, Deborah Grantham for assistance in conducting the experiment, Maria Ekstrand-Hurst and Otto Arnoscht for assistance in analyzing the data, and Janet D. Proctor for helpful comments on the manuscript. Requests for reprints should be sent to

Four types of stimulus-food contingencies were compared for effectiveness in producing and maintaining trial-stimulus pecking in pigeons. The four trial-stimulus contingencies were: (a) fixed-length trials contiguous with food, (b) variable-length trials contiguous with food, (c) fixed-length trials with fixed-length trace intervals preceding food, and (d) variable-length trials with variable-length trace intervals preceding food. In all cases the inter-trial interval was variable with a 30-sec. mean and the trial stimulus was 5 sec. or 5 sec. on the average. Trial-stimulus pecking was rapidly acquired when the trial stimulus was contiguous with food, whereas the trace procedures produced pecking in only a few pigeons and then only after considerable training. Neither variability in the trial-stimulus length nor in the trace interval reliably affected acquisition or maintenance.

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