Interpretability, validity, and the minimum important difference.

Patient-reported outcomes are increasingly used as dependent variables in studies regarding the effectiveness of clinical interventions. But patient-reported outcome measures (PROMs) do not provide intuitively meaningful data. For instance, it is not clear what a five point increase or decrease on a particular scale signifies. Establishing 'interpretability' involves making changes in outcomes meaningful. Attempts to interpret PROMs have led to the development of methods for identifying a minimum important difference (MID). In this paper, however, I draw on Charles Taylor's distinction between weak and strong evaluations to suggest that identifying a MID, specifically, a MID that uses a patient-reported reference group, may not provide an adequate interpretation of these measures. Moreover, I argue that the difficulty with interpreting these measures is tied to a larger problem concerning their validity. If researchers wish to interpret PROMs, they may first need to know more about the constructs they attempt to measure, namely, quality of life.