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In psychophysics, a just-noticeable difference, customarily abbreviated with lowercase letters as jnd, is the smallest detectable difference between a starting and secondary level of a particular sensory stimulus. It is also known as the difference limen or the differential threshold.
For many sensory modalities, over a wide range of stimulus magnitudes sufficiently far from the upper and lower limits of perception, the 'jnd' is a fixed proportion of the reference sensory level, and so the ratio of the jnd/reference is roughly constant (that is the jnd is a constant proportion/percentage of the reference level). Measured in physical units, we have
where is the original intensity of stimulation, is the addition to it required for the difference to be perceived (the jnd), and k is a constant. This rule was first discovered by Ernst Heinrich Weber (1795–1878), an anatomist and physiologist, in experiments on the thresholds of perception of lifted weights. A theoretical rationale (not universally accepted) was subsequently provided by Gustav Fechner, so the rule is therefore known either as the Weber Law or as the Weber–Fechner law; the constant k is called the Weber constant. It is true, at least to a good approximation, of many but not all sensory dimensions, for example the brightness of lights, and the intensity and the pitch of sounds. It is not true, however, of the wavelength of light. Stanley Smith Stevens argued that it would hold only for what he called prothetic sensory continua, where change of input takes the form of increase in intensity or something obviously analogous; it would not hold for metathetic continua, where change of input produces a qualitative rather than a quantitative change of the percept. Stevens developed his own law, called Stevens' Power Law, that raises the stimulus to a constant power while, like Weber, also multiplying it times a constant factor in order to achieve the perceived stimulus.
The jnd is a statistical, rather than an exact quantity: from trial to trial, the difference that a given person notices will vary somewhat, and it is therefore necessary to conduct many trials in order to determine the threshold. The jnd usually reported is the difference that a person notices on 50% of trials. If a different proportion is used, this should be included in the description—for example one might report the value of the "75% jnd".
Modern approaches to psychophysics, for example signal detection theory, imply that the observed jnd, even in this statistical sense, is not an absolute quantity, but will depend on situational and motivational as well as perceptual factors. For example, when a researcher flashes a very dim light, a participant may report seeing it on some trials but not on others.
The jnd formula has an objective interpretation (implied at the start of this entry) as the disparity between levels of the presented stimulus that is detected on 50% of occasions by a particular observed response (Torgerson, 1958), rather than what is subjectively "noticed" or as a difference in magnitudes of consciously experienced 'sensations.' This 50%-discriminated disparity can be used as a universal unit of measurement of the psychological distance of the level of a feature in an object or situation and an internal standard of comparison in memory, such as the 'template' for a category or the 'norm' of recognition (Booth & Freeman, 1993). The jnd-scaled distances from norm can be combined among observed and inferred psychophysical functions to generate diagnostics among hypothesised information-transforming (mental) processes mediating observed quantitative judgments (Richardson & Booth, 1993).
Marketing applications of the j.n.d. 
Weber’s law has important applications in marketing. Manufacturers and marketers endeavor to determine the relevant j.n.d. for their products for two very different reasons:
- so that negative changes (e.g. reductions in product size or quality, or increase in product price) are not discernible to the public (i.e. remain below j.n.d.) and
- so that product improvements (e.g. improved or updated packaging, larger size or lower price) are very apparent to consumers without being wastefully extravagant (i.e. they are at or just above the j.n.d.).
When it comes to product improvements, marketers very much want to meet or exceed the consumer’s differential threshold; that is, they want consumers to readily perceive any improvements made in the original products. Marketers use the j.n.d. to determine the amount of improvement they should make in their products. Less than the j.n.d. is wasted effort because the improvement will not be perceived; more than the j.n.d. is again wasteful because it reduces the level of repeat sales. On the other hand, when it comes to price increases, less than the j.n.d. is desirable because consumers are unlikely to notice it.
- Weber's Law of Just Noticeable Difference, University of South Dakota: http://people.usd.edu/~schieber/coglab/WebersLaw.html
- Booth, D.A., & Freeman, R.P.J. (1973). Discriminative measurement of feature integration. Acta Psychologica (Amsterdam).
- Richardon, N., & Booth, D.A. (1993). Acta Psychologica (Amsterdam).
- Torgerson, W.S. (1958). Theory and method of measurement. New York: Wiley.