The speed-accuracy decomposition (SAD) technique was developed by Meyer et al. (Psychol. Rev. 95 (1988) 183) for studying the course of information accumulation during stimulus processing. This technique aims at calculating the so-called sophisticated-guessing probability. In this note, it is shown that Meyer et al. (Psychol. Rev. 95 (1988) 183) used an incorrect formula for calculating the sophisticated-guessing probability. The correct formula is derived and the implications for the SAD technique are discussed.