Evidence-based assessment/Reliability

=Reliability=

This page focuses on psychometric reliability in the context of Evidence-Based Assessment. There are other more general and comprehensive discussions of reliability on Wikipedia and elsewhere.

Reliability describes the reproducibility of the score. It is expressed as a number that ranges from 0 (no reliable variance) to 1.0 (perfect reliability). Conceptually, the number can be thought of as the correlation of the score with itself. In classical test theory, it is the correlation of the observed score with the "true" score.

There are different facets of reliability. Internal consistency looks at whether different parts of the scale measure the same thing. One way of looking at internal consistency would be to split the items into two halves randomly, and then look at the correlation between the two scores (split-half reliability). Cronbach's alpha is the most widely used form of internal consistency reliability. Conceptually, alpha is the average of all possible split-half versions. Internal consistency is the most widely reported form of reliability because it is the most convenient and least expensive to estimate... not because it is always the most appropriate choice.