Just-noticeable difference
Also known as: Difference threshold
The smallest change in a stimulus a person can reliably detect.
What it means
The just-noticeable difference is the minimum change in a stimulus required for an observer to perceive a difference, conventionally defined as the change detected on a set proportion of trials. Weber's law holds that this threshold is a roughly constant fraction of the baseline magnitude — the Weber fraction — so it scales with stimulus size. The JND is a cornerstone measure of psychophysics and a practical tool for product and pricing design, where firms keep changes just below the JND to go unnoticed or push them just above to be perceived. It depends on the sensory dimension and on individual sensitivity. It operationalizes the limits of discrimination that the Weber–Fechner law formalizes.
Examples
A price nudged from $9.99 to $10.19 may fall below shoppers' JND and pass unnoticed, while a jump to $11.99 does not.
A chocolate bar quietly loses five grams and nobody says a word; lose thirty and shoppers write to the company — the same trick, two sides of the threshold.
Nudge a phone's brightness one step in a dim room and you see it instantly; the identical step in bright sunlight passes unnoticed, because the threshold scales with the baseline.
First described in Ernst Weber (1834); formalized in Fechner's psychophysics (1860).