Reliable change and the reliable change index: still useful after all these years?

In 1984 Jacobson and colleagues introduced the concept of reliable change, viz the amount of change on a measure that an individual needed to show to determine that it exceeded the extent of change likely due to measurement error alone. Establishing reliable change was a pre-requisite for determining clinical significance. This paper summarizes the rationale for determining reliable change as providing an individual-focused, idiographic alternative to the dominant nomothetic approach to clinical outcome research based on group mean data and statistical significance. The conventional computational steps for calculating an individual's standardized difference (reliable change) score and the minimum raw change score on the measure (a reliable change index) required to classify individuals as reliably positively changed, indeterminate, or reliably deteriorated are described. Two methods for graphically representing reliable change are presented, and a range of possible uses in both research and practice settings are summarized. A number of issues and debates concerning the calculation of reliable change are reviewed. It is concluded that the concept of reliable change remains useful for both cognitive behavioural researchers and practitioners, but that there are options regarding methods of computation. In any use of reliable change, the rationale for selecting among method options and the exact computations used need clear and careful description so that we can continuously judge the utility and appropriateness of the use of reliable change and enhance its value to the field. Key learning aims Recognizing why the concept of reliable change and the reliable change index is still important. Understanding the conventional formulas for calculating reliable change and the reliable change index (the Jacobson-Truax (JT) method). Seeing key ways that both researchers and practitioners can use reliable change to improve both research and practice. Understanding how several issues and debates that have arisen concerning the estimation of reliable change (e.g. how to accommodate practice effects) have progressed. Recognizing that there are a range of ways that reliable change may be estimated, and the need to provide full details of the method used in any particular instance of its use.