Reproducibility and responsiveness of evaluative outcome measures - Theoretical considerations illustrated by an empirical example

Objective: This article outlines basic methods for quantifying reproducibil ity and responsiveness of evaluative outcome measures. Methods: The background noise in stable patients provides the desired infor mation to quantify the reproducibility. From this, the smallest real differ ence (SRD) for longitudinal differences can be derived. We propose to use t he SRD to define responsiveness: from all patients who change according to an external criterion, we take the percentage that changes at least SRID on the outcome measure. A more complete picture of the responsiveness of the outcome measure arises when the receiver operating characteristic (ROC) is constructed, corresponding to all possible cut-off change scores. The propo sed methods are illustrated with an empirical example. Results. In the illustration the methods appeared to be very useful and com plemented each other. We could evaluate whether the observed change score w as larger than that expected due to chance. With the methods it was possibl e to evaluate both the ability of an instrument to detect change if there i s a real change in health status (sensitivity to change) and the ability to detect absence of change if there is no real change (specificity to change ). Conclusion: We presented the use of SRDs and ROC curves for quantifying rep roducibility and responsiveness. We started with the basic notions and arri ved at methods that are both understandable and useful.