Ensuring Test Reliability According to classical test theory, test reliability is based on the notion that test score reliability is comprised of two parts: true scores and error. A true score is an expected score on a test over an infinite number of testing instances; it is a theoretical idea that can never be known for sure. Errors are inaccuracies that make actual (observed) test scores differ from true scores. There are several different ways to measure a test’s reliability. Test-retest reliability looks at the correlation between original test administrations and re-tests. The span of time between the two administrations should be less that the time for the true scores to vary. Test-retest reliability looks at error due to time. Alternate-form reliability looks at the correlation between two different versions of a test. Split-half reliability is similar to alternative-form reliability, splitting a single test into two halves—usually odd and even…    read more