Just noticeable difference is proportional to the reference magnitude

What it is

Weber's law states that, under particular conditions, the smallest detectable change is approximately a constant proportion of the reference stimulus. This minimum detectable change is a just noticeable difference (JND). It concerns perceptual thresholds, not a complete account of how every change feels.

Why it happens

Perception often encodes relative difference: a small increment crosses noise at a low reference, while the same increment is absorbed by a high background. JND therefore grows with the reference. Weber fractions vary by sense, dimension, and range, and are affected by adaptation, attention, and presentation.

Studying it

Use two-alternative comparisons, staircase procedures, or constant stimuli to vary reference and increment and estimate a threshold at a chosen accuracy level. Randomise order, control adaptation and interval, and report absolute and relative thresholds. Interface studies can build separate detection curves for brightness, size, numeric, or progress change.

Where it stops holding

The relation is usually approximate in a middle range and deviates near sensory thresholds or saturation. Semantic meaning, colour contrast, motion, and task attention can make an interface change salient beyond physical proportion. One Weber fraction cannot be transferred across dimensions, devices, and populations.

Applying it

• Judge changes by relative magnitude, not only absolute number; adding one to a small and a large count has different visibility.
• For state changes that must be noticed, combine sufficiently large proportional difference with redundant text, position, or motion cues.
• Run detection tests on target displays and lighting to find the update threshold users actually miss.

Related

• Same group: B1.08.2 A small absolute change can be invisible against a large reference · B1.08.3 Progress and numeric changes should be designed proportionally, not only absolutely
• Nearby: B1.07 Power law of practice · B1.04 Hick-Hyman law
• Search terms: Weber's law · just noticeable difference · Weber fraction