The Difference Threshold and Signal-detection Theory
How much weight must be added to or subtracted from a stack of books for the carrier to sense that the load is heavier or lighter? The just-noticeable difference (JND) is the smallest change in sensation that a person is able to detect 50 percent of the time. The difference threshold is a measure of the smallest increase or decrease in a physical stimulus that is required to produce the JND. A person holding a 5-pound load would notice the addition of a single pound, but a person already holding 100 pounds would not be able to sense an additional pound. More than 150 years ago, researcher Ernst Weber(1795-1878)observed that the JND for all the senses depends on a proportion or percent of change rather than a fixed amount of change. This observation became known as Weber’s law. A weight being held must increase or decrease by 2 percent for the difference to be noticed. According to Weber’s law, the greater the original stimulus, the more it must be increased or decreased for the difference to be noticeable.
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