Evidence that the same-different disparity in letter matching is not attributable to response bias

Abstract

When pairs of letters or letter strings are judged as same or different, correct same responses are usually faster than correct different responses. Recently, Ratcliff and Hacker (1981, 1982) have argued that this same-different disparity is likely attributable solely to a bias to respond same. The present article reports an experiment in which the relative bias to respond same or different was varied for matches of single-letter pairs. Bias was manipulated by having 80, 60, 40, or 20 pairs in a block of 100 be same, with the remainder being different. For both sequential and simultaneous presentation of the letters, same reaction times had an overall advantage over different reaction times that could not be attributed to response bias. Moreover, consistent with Proctor's (1981) facilitation principle, this reaction time disparity was greater with sequential presentation than with simultaneous presentation. The larger reaction time advantage for same pairs with sequential presentation was obtained without an increase in the relative number of false-same responses, supporting the view that the facilitation is attributable to changes in sensitivity and not to changes in encoding criteria. © 1983 Psychonomic Society, Inc.