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THURSTONE'S LAW OF COMPARATIVE JUDGMENT. The American psychologist and psychometrician Louis Leon Thur-stone (1887-1955) formulated a mathematical model, called the law of comparative judgment, based on the scaling principle used by the German psychologist Gustav Fechner (1803-1887), which states that regardless of the physical values involved, stimulus differences that are detected equally-often are subjectively equal. The law of comparative judgment refers to a participant's perception of how two or more stimuli compare on a particular dimension and applies to the scaling of attributes (such as "beauty") for which there are no specifiable physical correlates or physically specifiable attributes. The law employs the psychophysical method of "paired comparisons" that was first introduced in 1894 by J. Cohn in his study of color preferences and then developed further by Thurstone. The paired-comparisons method is regarded as the most appropriate way of obtaining subjective/ value judgments. In the traditional method of paired-comparisons, every object in a set is presented for judgment in a pairwise fashion with every other object in the set (e.g., comparing tones according to "loudness," paintings according to "beauty," odors according to "pleasantness," and faces according to "similarity"). The power in the approach derives from the application of multidimensional and factor-analytic techniques to the data to reveal the underlying dimensions along which the judgments were made. Thurstone's law of comparative judgment makes it possible to obtain perceptual scale values associated with a single stimulus by starting quantitatively with individual difference measures. His law of the psychological distance between stimuli is cast in an equation involving a standard (z) score, variances, standard deviations, and a correlation coefficient. However, because experimental data on the values in the equation are not available usually, Thurstone's law of comparative judgment equation cannot be tested directly. The American psychometri-cian Warren S. Torgerson (1924-1999) showed how the theoretical principles of Thur-stone's law of comparative judgment are applicable to the special case of psychometric rating scales and data (cf., Galton, 18791880), and proposed a law of categorical judgment, which holds that an individual's psychological continuum can be divided into a specified number of ordered categories or steps. Torgersons's law of categorical judgment ideally should provide a scale with equal intervals for psychological measurement. Unfortunately, it is very difficult to obtain stable, useful, and valid ratings at even an ordinal scale level due to various biases in the observers and variability in the objects or persons rated. It is almost always necessary to provide the raters with a common anchor or reference point in order to obtain reliable data in such rating situations. See also DECISIONMAKING THEORIES; MEASUREMENT THEORY; PSYCHOPHYSICAL LAWS/ THEORY. REFERENCES

Galton, F. (1879-1880). Psychometric experiments. Brain, 2, 149-162. Cohn, J. (1894). Experimentelle Untersuchungen uber die gefuhlsbetonung der färben, helligkeiten, und ihrer com-binagtionen. Philosophische Studien, 10, 562-603. Thurstone, L. L. (1927). A law of comparative judgment. Psychological Review, 34, 273-286. Torgerson, W. S. (1958). Theory and method of scaling. New York: Wiley. Thurstone, L. L. (1959). The measurement of values. Chicago: University of Chicago Press.

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Brain Blaster

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