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The reliability coefficient

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The reliability coefficient is unlike other measures of correlation in that it is a quantitative statement of an act of judgment,—usually the test maker's, — that the things correlated are similar measures. Attempts to divorce it from this act of judgment are misdirected, just as would be an attempt to eliminate judgment of sameness of function of items when a test is originally drawn up. A “coefficient of cohesion,” entirely devoid of judgment, measuring the singleness of test function is proposed as an essential datum with reference to a test, but not as a substitute for the similar-form reliability co-efficient.

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Kelley, T.L. The reliability coefficient. Psychometrika 7, 75–83 (1942).

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