Louis Leon Thurstone

Louis Leon Thurstone (29 May 1887–29 September 1955) was a pioneer in the fields of psychometrics and psychophysics. He conceived the approach to measurement known as the law of comparative judgment, and is well known for his contributions to factor analysis.

Background and history
Thurstone originally received a masters in Mechanical Engineering from Cornell University in 1912, before undertaking a PhD in Psychology at the University of Chicago, which he completed in 1917. He later returned to that university, and he taught and conducted research there between 1924 and 1952. He is responsible for the standardized mean and standard deviation of IQ scores used today, as opposed to the mental age system originally used by Binet. He is also known for the development of the Thurstone scale.

Thurstone was also an environmentalist, and suggested an early system for generating hydroelectric power from rivers and waterfalls.

Factor analysis and work on intelligence
Thurstone's work in factor analysis led him to formulate a model of intelligence center around "Primary Mental Abilities" (PMAs), which were independent group factors of intelligence that different individuals possessed in varying degrees. He opposed the notion of a singular general intelligence that factored into the scores of all psychometric tests. This idea was unpopular at the time due to its obvious conflicts with Spearman's "mental energy" model, and is today still largely discredited. Nonetheless, Thurstone's contributions to methods of factor analysis have proved invaluable in establishing and verifying later psychometric factor structures.

Contributions to measurement
Despite his contributions to factor analysis, Thurstone (1959, p. 267) cautioned: "When a problem is so involved that no rational formulation is available, then some quantification is still possible by the coefficients of correlation of contingency and the like. But such statistical procedures constitute an acknowledgement of failure to rationalize the problem and to establish functions that underlie the data.  We want to measure the separation between the two opinions on the attitude continuum and we want to test the validity of the assumed continuum by means of its internal consistency". Thurstone's approach to measurement was termed the law of comparative judgment. He applied the approach in psychophysics, and later to the measurement of psychological values. The so-called 'Law', which can be regarded as a measurement model, involves subjects making a comparison between each of a number of pairs of stimuli with respect to magnitude of a property, attribute, or attitude. Methods based on the approach to measurement can be used to estimate such scale values.

Thurstone's Law of comparative judgment has important links to modern approaches to social and psychological measurement. In particular, the approach bears a close conceptual relation to the Rasch model (Andrich, 1978), although Thurstone typically employed the normal distribution in applications of the Law of comparative judgment whereas the Rasch model is a simple logistic function. Thurstone anticipated a key epistemological requirement of measurement later articulated by Rasch, which is that relative scale locations must 'transcend' the group measured; i.e. scale locations must be invariant to (or independent of) the particular group of persons instrumental to comparisons between the stimui. Thurstone (1929) also articulated what he referred to as the additivity criterion for scale differences, a criterion which must be satisfied in order to obtain interval-level measurments.