Jnd"s vary by sense & the smallest detectable difference is a fairly stable. The constant proportion is called the weber fraction and applies to all senses. Psychophysical scaling produce just noticeable differences in the magnitude of sensation: fechner"s law asserts that larger and larger increases in stimulus intensity are required to. Constant increments in stimulus intensity produce smaller and smaller increases. Perceptions can"t be measured on absolute scales, everything is relative. in the perceived magnitude of sensation. Proposes that detection of stimuli involves decision processes as well as sensory processes, which are both influenced by a variety of factors besides stimulus intensity. Response to a stimulus of a specific nature is pre set by a criterion which involves. Performance is also dependant on the amount of noise in the system as it becomes higher mental processing and expectations rather then raw sensations more difficult to pick up a weak system when there is more noise.