Limitations of “pragmatic” trials

Clinical trials testing the value of therapeutic interventions range between the two extremes of maximal internal or external validity. The former study, often called an efficacy trial, tests a treatment under optimal (that is, near laboratory) conditions, in which features such as the patient sample, the intervention, and the outcome measures are narrowly defined. Their research question is usually whether the treatment is efficacious when used under optimal conditions. Unfortunately this can mean that the findings are not generalisable—that is, the results of such studies cannot necessarily be transferred to the wider population. At the other end of the spectrum, pragmatic (practical or effectivity) trials are aimed to approximate the reality of clinical practice. Pragmatic trials have been defined as “trials for which the hypothesis and study design are formulated based on information needed to make a decision”.1 The test treatment is often compared with clinically relevant interventions or with no treatment rather than with a placebo. Alternatively, the test treatment, in addition to usual treatment, is compared with usual (often diverse) treatment alone. Other features of pragmatic trials include diverse study populations and a range of outcome measures.1