The just noticeable difference, also known as the difference threshold, is the smallest possible difference between two stimuli that can be detected at least half the time. According to Weber’s law, this difference threshold is a constant proportion of the original threshold size.
Weber’s law, named for German physiologist Ernst Weber, is a principle of perception which states that the size of the just noticeable difference varies depending upon its relation to the strength of the original stimulus.
Observations About Weber’s Law
“Whether we can detect a change in the strength of a stimulus depends on the intensity of the original stimulus. For example, if you are holding a pebble (the original stimulus), you will notice an increase in weight if a second pebble is placed in your hand. But if you start off holding a very heavy rock (the original stimulus), you probably won’t detect an increase in weight when the same pebble is balanced on it. What Weber’s law underscores is that our psychological experience of sensation is relative. There is no simple, one-to-one correspondence between the objective characteristics of a physical stimulus, such as the weight of a pebble, and our psychological experience of it” (Hockenbury & Hockenbury, 2007)
Hockenbury, D. H., & Hockenbury, S. E. (2007). Discovering psychology. New York: Worth Publishers.