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A basis for analyzing test-retest reliability

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Three sources of variation in experimental results for a test are distinguished: trials, persons, and items. Unreliability is defined only in terms of variation over trials. This definition leads to a more complete analysis than does the conventional one; Spearman's contention is verified that the conventional approach—which was formulated by Yule—introduces unnecessary hypotheses. It is emphasized that at least two trials are necessary to estimate the reliability coefficient. This paper is devoted largely to developinglower bounds to the reliability coefficient that can be computed from but asingle trial; these avoid the experimental difficulties of making two independent trials. Six different lower bounds are established, appropriate for different situations. Some of the bounds are easier to compute than are conventional formulas, and all the bounds assume less than do conventional formulas. The terminology used is that of psychological and sociological testing, but the discussion actually provides a general analysis of the reliability of the sum ofn variables.

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The writer is indebted to the members of his statistical seminar, to Professor Mark Kac, and to Professor Samuel A. Stouffer and his staff in the Research Branch, Information and Education Division, War Department, for their helpful comments on this paper.

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Guttman, L. A basis for analyzing test-retest reliability. Psychometrika 10, 255–282 (1945).

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