Thorndike and Wood

A basic puzzle in the history of measurement is how the "test score tradition" became dominant in education and psychology? Cattell (1893) and Thorndike (1904) provided solid foundations for the measurement models that would be recognized today as "item response theory". Thurstone (1925, 1926) also provided a rich source that was ignored by educational and psychological testers. When I compare Thorndike, Thurstone and Rasch (Engelhard 1984, 1991), I am impressed by the conceptual and empirical correspondence among these theorists. E.L. Thorndike is the father of item response theory.

In pursuit of how the test score tradition became dominant, I have been studying Thorndike and Wood. Thorndike's work is well known. Ben Wood's is not. But Wood was a driving force behind the measurement movement of the 1920's that replaced essay examinations with multiple-choice items (Downey 1965; Saretzky 1989). Wood completed his dissertation under Thorndike at Teachers College. My plan was to use Wood's work to understand Thorndike. But analysis of Wood's exposition of Thorndike's theory made it apparent that, despite a nominal relationship, Wood's work diverged noticeably from Thorndike's. Even though Wood studied with Thorndike and considered himself Thorndike's heir, the differences between their measurement theories provide a potent example of the consequences of different research traditions. Thorndike was in the scaling tradition. Wood was in the test score tradition - the tradition that has dominated testing practices during the 20th century.

Both Thorndike and Wood used the word "objectivity" as a cornerstone of educational measurement. Thorndike defined an abstract "objective scale" as one on which all competent thinkers agree. To increase agreement, Thorndike envisioned the construction of measuring sticks that provided explicit definitions of the scales. He advocated the construction of what would be recognized today as variable maps. Thorndike constructed these maps by calibrating agents in a variety of content areas: handwriting, written composition and vocabulary.

Wood, on the other hand, defined "objectivity" in a more limited manner. Wood's "objectivity" was a matter of how a test was scored. "The true-false test is a good example of an objective mental scale. No competent person would disagree in rating a true-false paper, provided they used the key which accompanies the test." Wood, however, did recognize that "it is perfectly possible to have a very objective scale without having one which measures the facts to be measured" - foreshadowing the injunction in most introductory measurement texts that reliability is necessary but not sufficient for establishing validity.

Thorndike defined "objectivity" in a broad sense that included aspects of reliability and validity. Wood's alternative treatment of "objectivity" as simply a problem of reliability is a defining characteristic of the test score tradition. Traditional classical test theory (CTT), rooted in Spearman (1904), is preoccupied with reliability. A negative consequence is that validity becomes a separate issue that leads to an "attenuation paradox": as a test becomes more reliable, the validity of the test scores as measured by correlation with a criterion variable becomes smaller.

Thorndike and Wood each defined a measurement problem called "objectivity" and sought solutions in ways that were consistent with the research traditions within which they operated. Wood believed that he was developing an exegesis of Thorndike's theory, but this was not the case for most measurement problems addressed by these men. Write to me, if you are interested in more detail on Thorndike and Wood.

Cattell JK 1893. Mental Measurement. Philosophical Review 2 316-332

Downey MT 1965. Ben D. Wood: Educational reformer. Princeton, NJ: Educational Testing Service

Engelhard G 1984. Thorndike, Thurstone and Rasch: A comparison of their methods of scaling psychological tests. Applied Psychological Measurement 8 21-38

Engelhard G 1991. Thorndike, Thurstone and Rasch: A comparison of their approaches to item-invariant measurement. Journal of Research and Development in Education 24(2) 45-60

Engelhard G (in press). Historical views of invariance: Evidence from the measurement theories of Thorndike, Thurstone and Rasch. Educational and Psychological Measurement

Saretzky GD 1989. A guide to the Ben D. Wood papers. Princeton, NJ: ETS Archives

Spearman C 1904. General intelligence, objectively determined and measured. American Journal of Psychology 15 201-293

Thorndike EL 1904. An introduction to the theory of mental and social measurements. New York: Teachers College, Columbia University

Thurstone LL 1925. A method of scaling psychological and educational tests. Journal of Educational Psychology 16 433-451

Thurstone LL 1926. The scoring of individual performance. Journal of Educational Psychology 17 446-457



Thorndike and Wood, G Engelhard Jr. … Rasch Measurement Transactions, 1991, 5:2 p. 146




Rasch Publications
Rasch Measurement Transactions (free, online) Rasch Measurement research papers (free, online) Probabilistic Models for Some Intelligence and Attainment Tests, Georg Rasch
Applying the Rasch Model 2nd. Ed., Bond & Fox Best Test Design, Wright & Stone Rating Scale Analysis, Wright & Masters
Introduction to Rasch Measurement, E. Smith & R. Smith Introduction to Many-Facet Rasch Measurement, Thomas Eckes Invariant Measurement: Using Rasch Models in the Social, Behavioral, and Health Sciences, George Engelhard, Jr.
Statistical Analyses for Language Testers, Rita Green Rasch Models: Foundations, Recent Developments, and Applications, Fischer & Molenaar Journal of Applied Measurement

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