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

When pairs of letters or letter strings are judged assame ordifferent, correctsame responses are usually faster than correctdifferent responses. Recently, Ratcliff and Hacker (1981, 1982) have argued that thissame-different disparity is likely attributable solely to a bias to respondsame. The present article reports an experiment in which the relative bias to respondsame ordifferent was varied for matches of single-letter pairs. Bias was manipulated by having 80, 60, 40, or 20 pairs in a block of 100 besame, with the remainder beingdifferent. For both sequential and simultaneous presentation of the letters,same reaction times had an overall advantage overdifferent 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 forsame 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.