Partition scaling methods are designed to construct interval scales of psychological attributes directly from the judgments of the observers. In these methods, the observer is required to partition the psychological continuum into equal sensory intervals. To accomplish this objective, two main kinds of method, equisection scaling and category scaling, have been developed.
Equisection is a method which, as its name implies, requires observers to section the psychological continuum into equal sense distances. The psychological difference between two brightnesses, two loudnesses, or two sweetnesses are examples of sense distances. The observer's task is to report whether the sense distance between sensations A and B is less than, greater than, or equal to the sense distance between sensations C and D. The experimental problem is to discover the stimuli corresponding to a series of sensations separated by equal sense distances.
Bisection, the earliest version of the equisection approach to psychological scaling, was used by Plateau in the 1850s ( Plateau, 1872). In this method, two stimuli are presented for inspection; the observer is then asked to choose a third stimulus of intermediate value, so that the sense distance between the two end stimuli is divided exactly in half--thus resulting in two sense distances of equal size defined by three stimulus values. Plateau, for example, had artists paint a gray that was midway between black and white. The term equisection is generally reserved for