Individual Inconsistency and Reliability of Measurement1

THE concept of reliability of measurement is clearly not as simple and static as standard definitions often imply. Reliability is not an all or none criterion which, if once satisfied, is invariant for a given measuring instrument, for different groups, or for different testing conditions. Reliability may also be examined in relation to a given measure for a given individual, thus implying the relevance of examining specific individual factors contributing to unreliability. Unreliability, Thorndike’s “error variance” (1951), can be seen as being composed of two classes of elements: (1) characteristics of the observer and the environment; and (2) characteristics of the individual. The first group is composed of such factors as poor testing conditions, careless investigators, inaccurate calculations and numerous other factors which are external to the individual being examined. Included in individual characteristics are aspects such as test-taking ability, response sets, response styles and guessing habits. Reliability of measurement implies more than consistency of response over a time interval. Rather, reliability can be discussed in two different framemorks-test-retest reliability (stability) and internal consistency reliability. Test-retest reliability refers to the stability of measurement across some time interval. Stability de-