Response distributions in intensity resolution and speech discrimination

In this paper the assumption of an equal, Gaussian distribution of the response to each stimulus in an experiment, an assumption which has to be met if d' is to be estimated by calculating the difference between z(H) and z(FA), is tested for two different sets of stimuli: 1000-Hz tones differing in level only, and a continuum of stop consonants, obtained by full spectral interpolation between /p/, /t/, and /k/. Response distributions were measured directly by means of a form of non-numerical magnitude estimation, in which subjects had to indicate the position of each stimulus on a quasi-continuous rating scale. It could be shown that, in general, all distributions were sufficiently unimodal, but that their variances differed. The consequences for the calculation of d' are unlikely to be serious.