9.2.2 Method of constant stimuli
This method is similar to that described above but has two advantages over the method of limits. The first is that it's designed to overcome bias inherent in presenting stimuli in a set order. This is done by randomising the order of presentation of stimuli. The subject therefore has no way of anticipating the intensity of the next stimulus (it could be softer or louder than the preceding one). In the table, the stimuli would be presented in a random order: for example, 13 dB SPL, 17 dB SPL, 12.5 dB SPL, etc. until each of the intensities is presented a sufficient number of times. Once again a psychometric function is generated and the intensity of the stimulus value detected on 50 per cent of the trials is used as the measure of absolute threshold.
The method of constant stimuli has a second advantage over the method of limits in that the experimenter can get an estimate of the listener's bias by including ‘catch’ or ‘blank’ trials in the sequence. That is, occasionally the experimenter presents no stimulus to the listener. The listener does not know this has happened, and so the response is still ‘yes’ or ‘no’. If there is a bias towards either response, then the proportion of ‘yes’ responses in the blank trials will reflect this bias.