Weber's law

Meaning: Weber's law, also called the Weber-Fechner relationship in sensation and perception, describes difference threshold. The just noticeable difference (JND) is the smallest change in a stimulus that a person detects about half the time. The law states that the JND grows in proportion to the starting intensity. Adding a small weight to a light load is noticeable, but the same addition to a heavy load is not. Each sense has its own constant.

Equation: ΔI / I = k, where ΔI is the JND, the smallest detectable change in stimulus intensity (in the unit of that stimulus, such as g for weight or dB for loudness), I is the original stimulus intensity (same unit as ΔI), and k is Weber's constant, a unitless fraction specific to each sense.

On the MCAT: A passage gives a JND for one starting intensity, such as noticing 2 g added to 100 g. You then predict the JND for a different starting value. At 200 g the JND is about 4 g, because the ratio stays the same. A smaller k means finer discrimination.

Common confusion: Students treat the JND as a fixed amount. It is a fixed ratio. Weber's law is also not the same as absolute threshold, which is the minimum intensity detected at all. The law is an approximation that fails near the extremes of very faint or very intense stimuli.