The differences and overlaps between ‘explanatory’ and ‘pragmatic’ controlled trials: a historical perspective

Abstract

In designing a controlled trial, a potential tension may exist between (1) providing insights into therapeutic mechanisms and (2) guiding decisions about 'what works' in everyday practice. In their 1967 paper in the Journal of Chronic Diseases, Schwartz and Lellouch 1 designated these aims as 'explanatory' and 'pragmatic', illustrating the potential tension with the different trial designs needed to assess the effect of an agent hypothesised to increase responsiveness to radiotherapy (a 'radiosensitiser') with immediate initiation of radiotherapy in the 'pragmatic' design versus a 30-day delay in initiating radiotherapy for the 'explanatory' design. This dilemma is manifest in real clinical situations, such as chemotherapy or radiotherapy prior to cancer surgery ('neoadjuvant therapy'), 'pre-habilitation' to prepare patients for joint surgery and whether to use a placebo when comparing two versus four times daily drug dosing.Most study design choices are not so clearly and unidimensionally 'pragmatic' or 'explanatory'. The PRagmatic-Explanatory Continuum Indicator Summary (PRECIS) tool 2 was developed to provide a graphical assessment of this spectrum for 10 'domains' that include eligibility criteria, flexibility of interventions, practitioner expertise, intensity of follow-up, compliance with treatments and scope of the analysis.The 'explanatory' versus 'pragmatic' tensions can sometimes be resolved. We suggest three categories:(1) sometimes both 'explanatory' and 'pragmatic' objectives of a trial can be achieved by aligning the design to address both; (2) sometimes there are possible compromises, for example, using an 'explanatory' sub-study within the framework of a 'pragmatic' main trial; and (3) rarely, there are incompatible objectives as illustrated by Schwartz and Lellouch. 1 Both the 'explanatory' and 'pragmatic' aims of trials are important. In general, as research moves from an understanding of the mode of therapeutic action towards clinical application, studies move increasingly from the 'explanatory' mode to the 'pragmatic' mode. Recognising when and how to focus on 'explanatory' versus 'pragmatic' reasons for research will remain an important element in that judgement.

How can the NHS be funded?

“…When I recently discussed the state of the NHS with a Brazilian public health expert, I was reminded that the NHS was the model for many health systems around the world and continues to be influential. To the rest of the world, the NHS is a pragmatic experiment -albeit not in the scientific sense 1 -that they can learn from.…”