Typically, the classical textbooks on psychophysics, such as Titchene (1905), Brown and Thomson (1921), Wood worth (1938), and Guilford (1936, 1954) give the impression that the task of psychophysics is to measure these thresholds and that such measurements are subject to so many sources of "error" or variability that statistical calculations must be applied to the data in order to estimate the "true value" of the threshold. In all of these texts, for example, the measurement of the differential threshold from data collected by the method of right and wrong cases is discussed exhaustively. The following is an example of such an early use of probability theory as a device for estimating the "true value" of the threshold.