RESUMEN
This paper describes test theory, which is the theoretical foundation of learner assessment, in relation to its application in Common Achievement Tests. To be specific, classical test theory and the reliability coefficients, generalizability theory, and item response theory are taken up. In classical test theory, the observed score X is expressed as the sum of a true-score component T and an error component E. The ratio of the true-score variance to the observed score variance defines the reliability coefficient. Generalizability theory is an extension of the notion of reliability in classical test theory. Item response theory overcomes the limitations of classical test theory and can express the properties of items (difficulty and discrimination) and examinees’ abilities separately.