Active stimulus control by subjects can bias psychophysical measurements

Evidence

All the psychophysical methods discussed so far have the common feature that a final value of threshold or of ratio can be deduced from a single trial. In the first two methods described, the subject is actively involved in the task by controlling the stimulus. Sometimes such an activity may produce a bias, as for example in loudness comparisons. In such cases, the average of the results of the two measurements, one with varying sound “A” and another with varying sound “B” leads to the value of interest. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 113

Categorization

• QUESTION: What specific biases does active stimulus control introduce in loudness comparison tasks? Worth chasing for experimental design.