Scientific calculation of the average detectable difference between two measurable qualities, such as weight, brightness of light, or loudness of sound.
When we try to compare two different objects to see if they are the same or different on some dimension (e.g., weight), the difference between the two that is barely big enough to be noticed is called the just noticeable difference (JND). Just noticeable differences have been studied for many dimensions (e.g., brightness of lights, loudness of sounds, weight, line length, and others).
The human sensory system does not respond identically to the same stimuli on different occasions. As a result, if an individual attempted to identify whether two objects were of the same or different weight he or she might detect a difference on one occasion but will fail to notice it on another occasion. Psychologists calculate the just noticeable difference as an average detectable difference across a large number of trials in which the difference is noticeable half of the time. This means that the JND is not the difference required for the difference to be noticed each and every time. The JND does not stay the same when the magnitude of the stimuli change. In assessing heaviness, for example, the difference between two stimuli of 10 and 11 grams could be detected, but we would not be able to detect the difference between 100 and 101 grams. As the magnitude of the stimuli grow, we need a larger actual difference for detection. The percentage of change remains constant in general. To detect the difference in heaviness, one stimulus would have to be approximately 2% heavier than the other; otherwise, we will not be able to spot the difference.
For example, humans require a 4.8% change in loudness to detect a change while a 7.9% in brightness is necessary. Psychologists refer to the percentages that describe the JND as Weber fractions, named after Ernst Weber (1795–1878), a German physiologist whose pioneering research on sensation had a great impact on psychological studies.
Kellogg, Ronald. Fundamentals of Cognitive Psychology, 3rd edition. Thousand Oaks, CA: SAGE, 2016.
McBride, Dawn, and J. Cooper Cutting. Cognitive Psychology: Theory, Process, and Methodology. Los Angeles: SAGE, 2016.