Methods for assessing the reliability of measurement scales were investigated. Reliability, which is defined as the ratio of the true variance to the total variance, is an important property of measurement. In order to estimate the reliability, the concepts of measurement model and measurement scale are required. The model specifies the structure of the measurement, and the scale, which is a combination of the measured items, represents a realization of the theoretical notions. The focus was on two measures of reliability: Cronbach\'s alpha, which is widely applied, and Tarkkonen\'s more general measure.