Principals of Psychological Tests
The Concept of Reliability Reliability essentially refers to the consistency of scores obtained by the same person. It measures consistency when a person is re-examined with the same test on different occasions. The concept also applies when testing with equivalent sets of items or under varying conditions. Reliability underlies the computation of the error of measurement for a single individual’s score. It helps predict the range of fluctuation likely to occur due to irrelevant or unknown factors. Individual differences in scores are attributed to "true" differences and "chance errors". The crux of reliability lies in the definition of error variance—factors irrelevant to the test purpose. Examiners aim to reduce error variance by maintaining uniform and standard testing conditions. No psychological test is a perfectly reliable instrument; every test has some degree of error. A statement of reliability should always accompany a test to characterize its measurement ...