The oldest branch of experimental psychology, concerned with the relationship between the psychological magnitude of sensations and the physical intensity (broadly interpreted to include any perceptible stimulus dimension) of the stimuli that produce them. It is concerned partly with the determination of absolute thresholds and difference thresholds, using variations of three classical methods called the method of constant stimuli, the method of limits, and the method of average error, and partly with the establishment of psychophysical functions and psychophysical scales. Among its greatest achievements are Weber's law, Fechner's law, and the power law, and one of its most important modern forms is signal detection theory. See also absolute error, ABX paradigm, catch trial, constant error, cross-modal matching, direct scaling, fractionation, Fullerton-Cattell law, global psychophysics, indirect scaling, interval of uncertainty, loudness matching, magnitude estimation, magnitude production, Merkel's law, method of absolute judgement, method of bisection, method of equal and unequal cases, method of equal-appearing intervals, method of gradation, method of paired comparisons, olfactie, Piper's law, production method, relative error, Ricco's law, sense ratio method, staircase method, standard stimulus, unfolding technique, variable error, variable stimulus, Weber fraction. psychophysical adj.