Measurement outcomes are frequently used as evidence in favor of or against medical and surgical interventions, health policies, and system designs. Indeed, in the medical and health services research literature, outcomes are the currency of policy debate and decision making. Yet in the philosophy of science and philosophy of medicine, the measures used in evidence-based medicine (EBM) are rarely discussed. Rather, the focus here is almost exclusively on study design and hierarchies of evidence. This concentration on the methodology of study design has meant that for practical purposes the measures used in randomized controlled trials, observational studies, audits, and so forth, appear as a "black box." Yet as I argue in the first part of this article, an engagement with measurement can improve our understanding of EBM and the quality of our evidence. In the second part of the article, I develop such an engagement with one aspect of measurement, namely, the validity of patient-reported outcome measures. Here, I illustrate some of the complexity that is required to improve the validity of these measures and hence the validity of our study outcomes, that is, evidence. The concentration in philosophy of science on study design over measurement methodology perhaps reveals the interest that many philosophers of medicine have in causation, but there is more to the production of high-quality scientific evidence than securing the causal inference.
Keywords: evidence-based medicine; measurement; patient-reported outcomes; study design; validity.