Reliability as a central concept of test theory dates back to the beginning of the 20th century. It is based on the existence of intra-individual variability as well as variation between persons. With intra-individual variability or measurement error, true score was also introduced as a central concept of classical test theory. Observed score variance could then be thought of as true score variance plus error variance. The reliability of a test, rating scale, assessment or any other more or less standardized procedure within a given (sub)population of persons (or other objects of measurements, e.g. classrooms) is defined as the ratio of true score variance to observed score variance or as the squared correlation between observed scores and true scores (Lord & Novick, 1968: 61):


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