The topic of test reliability is about the relative consistency of test scores and other educational and psychological measurements. In this module, the idea of consistency is illustrated with reference to two sets of test scores. A mathematical model is developed to explain both relative consistency and relative inconsistency of measurements. A means of indexing reliability is derived using the model. Practical methods of estimating reliability indices are considered, together with factors that influence the reliability index of a set of measurements and the interpretation that can be made of that index.

Keywords:  classical test theory, CTT, congeneric, KR-20, KR-21, Cronbach’s alpha, Pearson correlation, reliability, Spearman-Brown formula, parallel, tau-equivalent, test-retest, validity