First we will look at indirect scaling. In indirect scaling, subjects are exposed to two stimuli of equal magnitude. One of the stimuli is made progressively greater/less than the other. The subject is asked to determine when the stimuli are no longer equal. This is called determination of the just noticeable difference. The participant is never required to judge the strength of a sensation, rather they are asked to detect differences between two stimuli (Allard, 2001). Weber showed that the same proportional change will always produce a just noticeable difference. This is expressed in Weber’s Law, K =sI/I, where the just noticeable difference is sI, K is a constant proportion and I is the standard stimulus. Gustav Fechner assumed that one just noticeable difference is perceptually equal to any other just noticeable difference, regardless of the absolute magnitude of the stimuli and derived an equation based on Weber’s Law (Allard, 2001). The relationship that Fechner theorized is formalized in the equation, S=KlogI, Fechner’s Law. S is the sensation, I is the stimulus magnitude and K is constant. 

    The second experimental type is direct scaling. In contrast to indirect scaling, in direct scaling, subjects are asked to directly judge the strength of a sensation induced by a stimulus (Allard, 2001). In a direct scaling experiment, a subject may be asked to judge the magnitude of his/her sensation on a set number scale (e.g., from 1 to 10) with different verbal anchors indicating what each number means. Instead of this method of direct scaling an experimenter may choose to use another method, magnitude estimation.