Validity by Numbers vs. Validity by Theory

Robert L. Ebel: "These observations suggest that the concept of validity itself may be weak scientifically. Most of the definitions of validity can be shown to be derived from the basic notion that validity is the degree to which a test measures what it is supposed to measure. But how does one know what a test is supposed to measure? On a superficial level, perhaps, it may be suggested by the test title - academic aptitude, mathematics achievement, or social studies background, for example - but these suggestions are by no means definitive.

"Does the criterion [i.e., definitive information outside the test] tell us what the test is supposed to measure? It might, if criteria were given to us. Usually they are not. They have to be devised, often after the test itself was constructed. Toops (1944) has said:

Possibly as much time should be spent in devising the criterion as in constructing and perfecting the test. This important part of a research seldom receives half the time or attention it requires or deserves. If the criterion is slighted the time spent on the tests is, by so much, largely wasted.

"The ease with which test developers can be induced to accept as criterion measures quantitative data [for test validation by correlation], having the slightest appearance of relevance to the trait being measured is one of the scandals of psychometry. To borrow a figure of speech from Thorndike (1922), they will use the loudness of the thunder as a criterion for their measurements of the voltage of the lightning."

Ebel R. L. (1961) Must all tests be valid? American Psychologist, 16, 640-7, reprinted in p.222-3 in W. A. Mehrens & R. L. Ebel (Eds.) Principles of Educational and Psychological Measurement. Chicago: Rand McNally. 1967.

Thorndike E.L. (1922) Measurement in education. 21st Yearbook, NSSE. Bloomington, Il: Public School Publishing Co.

Toops H. A. (1944) The criterion. Educ. Psychol. Meas., 4, 271-297.


Music is the art of measuring well.

Augustine of Hippo

Validity by Theory Ebel, R.L. Validity by Numbers vs. … Rasch Measurement Transactions, 1999, 13:3 p. 713




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