Abstract
Abstract Reliability, ρ XX ′ , is the degree to which test scores are precise and free from error. Classical test theory defines reliability as the ratio of true score variance to observed score variance: . This article provides a comprehensive definition of reliability and then describes the methods by which reliability is commonly estimated, including their limitations. The article then discusses several closely‐related topics: the standard error of measurement; reliability in subpopulations; the Spearman‐Brown prophesy formula; true score correlations; reliability of speeded tests; and the relationship between classical reliability and item response theory information.
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