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The method of constant stimuli(also known as the method of right and wrong cases or frequency method) was one of three psychophysical methods for studying sensory absolute and differential thresholds developed by Gustav Fechner. See method of limits and method of adjustment for the other methods
The absolute threshold of a modality is examined by repeatedly presenting a series of fixed stimuli in random order and asking the subject to report if they detect it or not. The threshold is determined as the value that was detected 50% of the time.
A differential threshold is explored by repeatedly presenting a series of fixed stimuli in random order and asking the observer to report on the size relative to a comparison stimulus. The threshold is determined to be the point at which the subject notes a just noticeable difference from the comparison stimulus on 50% of the trials