An "exhaustive" search for tasks that fall between the choice and simple reactions.

An experimental search was made for reaction time (RT) tasks that require the processing operation of stimulus discrimination or response selection, but not both. The existence of such a task would allow the subtractive determination of the mean duration of the separate operations. Each of the three reactions that historically have been represented as possessing only one of the two operations was subjected to empirical testing. None of them met the most basic requirements. Donders' (1868/1969) contingent (c) reaction and Taylor's (1966) selection (b')reaction were not reliably faster than a compatible choice (b) reaction. Wundt's (1880) discrimination (d) reaction was not sensitive to difficulty of discrimination. The possibility is suggested that stimulus discrimination and response selection in the choice reaction are inseparable operations. The present negative results emphasize the importance of establishing validity of any use of the subtraction method through appropriate experimental methods. Some parallels of the compatibility issue are found in experiments in which RT is used as a measure of the duration of programming operations.