Rosenblith and Stevens obtained larger frequency limen by the ABX method than by the AX method. The ratio of the limens is here calculated on the assumption that there is a statistical variation in the physiological or psychological response to a given stimulus. The ratio of limens is 2.12 for a Gaussian distribution of response and 2.14 for a uniform distribution. The model fits data for subject KNS well, but not the data for subject WAR.