Louis Leon Thurstone made significant contributions in many areas of psychology, including psychometrics, statistics, and the study of human intelligence. He developed methods for scaling psychological measures, assessing attitudes, and test theory, among many other influential contributions. He is best known for the development of new factor analytic techniques to determine the number and nature of latent constructs within a set of observed variables.

The new statistical techniques developed by Thurstone provided the necessary tools for his most enduring contribution to psychology: The Theory of Primary Mental Abilities, a model of human intelligence that challenged Charles Spearman’s then-dominant paradigm of a unitary conception of intelligence. Spearman, using an earlier approach to factor analysis, found that scores on all mental tests (regardless of the domain or how it was tested) tend to load on one major factor. Spearman suggested that these disparate scores are fueled by a common metaphorical “pool” of mental energy. He named this pool the general factor, or (Spearman, 1904).

Thurstone argued that g was a statistical artifact resulting from the mathematical procedures used to study it. Using his new approach to factor analysis, Thurstone found that intelligent behavior does not arise from a general factor, but rather emerges from seven independent factors that he called primary abilities: word fluency, verbal comprehension, spatial visualization, number facility, associative memory, reasoning, and perceptual speed (Thurstone, 1938). Furthermore, when Thurstone analyzed mental test data from samples comprised of people with similar overall IQ scores, he found that they had different profiles of primary mental abilities, further supporting his model and suggesting that his work had more clinical utility than Spearman’s unitary theory. However, when Thurstone administered his tests to an intellectually heterogeneous group of children, he failed to find that the seven primary abilities were entirely separate; rather he found evidence of g. Thurstone managed an elegant mathematical solution that resolved these apparently contradictory results, and the final version of his theory was a compromise that accounted for the presence of both a general factor and the seven specific abilities. This compromise helped lay the groundwork for future researchers who proposed hierarchical theories and theories of multiple intelligences (Ruzgis, 1994).

The Theory of Primary Mental Abilities

Thurstone (1938) proposed a theory of primary mental abilities. Although this theory is not widely used today, the theory forms the basis of many contemporary theories, including two contemporary theories discussed later, those of Gardner (1983) and Carroll (1993). It is also the basis for many contemporary group tests of intelligence.

Thurstone (1938) analyzed the data from 56 different tests of mental abilities and concluded that to the extent that there is a general factor of intelligence, it is unimportant and possibly epiphenomenal. From this point of view there are seven primary mental abilities:

·       Verbal comprehension. This factor involves a person’s ability to understand verbal material. It is measured by tests such as vocabulary and reading comprehension.

·       Verbal fluency. This ability is involved in rapidly producing words, sentences, and other verbal material. It is measured by tests such as one that requires the examinee to produce as many words as possible beginning with a particular letter in a short amount of time.

·       Number. This ability is involved in rapid arithmetic computation and in solving simple arithmetic word problems.

·       Perceptual speed. This ability is involved in proofreading and in rapid recognition of letters and numbers. It is measured by tests such as those requiring the crossing out of As in a long string of letters or in tests requiring recognition of which of several pictures at the right is identical to the picture at the left.

·       Inductive reasoning. This ability requires generalization—reasoning from the specific to the general. It is measured by tests, such as letter series, number series, and word classifications, in which the examinee must indicate which of several words does not belong with the others.

·       Spatial visualization. This ability is involved in visualizing shapes, rotations of objects, and how pieces of a puzzle fit together. An example of a test would be the presentation of a geometric form followed by several other geometric forms. Each of the forms that follows the first is either the same rotated by some rigid transformation or the mirror image of the first form in rotation. The examinee has to indicate which of the forms at the right is a rotated version of the form at the left, rather than a mirror image.

Today, Thurstone’s theory is not used as often in its original form, but it has served as a basis for many subsequent theories of intelligence, including hierarchical theories and modern theories such as Gardner’s (1983). Thus, to the extent that a theory is judged by its heuristic value, Thurstone’s has been one of the most important in the field.

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 stimuli. 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 measurements.

Comparative Judgment Scale

In psychology, the 'Thurstone scale' was the first formal technique for measuring an attitude. It was developed by Thurstone in 1928, as a means of measuring attitudes towards religion. It is made up of statements about a particular issue, and each statement has a numerical value indicating how favorable or unfavorable it is judged to be. People check each of the statements to which they agree, and a mean score is computed, indicating their attitude.

This methodological contribution of Thurstone has been noted as one of the first attempts at developing a comparative judgment scaling technique. This method of measuring attitudes on an interval scale allowed statements related to an attitude to be ranked in reference to each other. The extreme opposites of the attitude and the opinions representing the equally-distanced steps in between the opposites could be established.

This rank scale can be used to rank all possible feelings related to an issue and to categorize people expressing an opinion based on the rank of that opinion. It is used today mainly in basic research. Most researchers acknowledge that, while it is very accurate, it is too complex for applied settings.