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, 11 (3), e0150205

Defining Feasibility and Pilot Studies in Preparation for Randomised Controlled Trials: Development of a Conceptual Framework


Defining Feasibility and Pilot Studies in Preparation for Randomised Controlled Trials: Development of a Conceptual Framework

Sandra M Eldridge et al. PLoS One.


We describe a framework for defining pilot and feasibility studies focusing on studies conducted in preparation for a randomised controlled trial. To develop the framework, we undertook a Delphi survey; ran an open meeting at a trial methodology conference; conducted a review of definitions outside the health research context; consulted experts at an international consensus meeting; and reviewed 27 empirical pilot or feasibility studies. We initially adopted mutually exclusive definitions of pilot and feasibility studies. However, some Delphi survey respondents and the majority of open meeting attendees disagreed with the idea of mutually exclusive definitions. Their viewpoint was supported by definitions outside the health research context, the use of the terms 'pilot' and 'feasibility' in the literature, and participants at the international consensus meeting. In our framework, pilot studies are a subset of feasibility studies, rather than the two being mutually exclusive. A feasibility study asks whether something can be done, should we proceed with it, and if so, how. A pilot study asks the same questions but also has a specific design feature: in a pilot study a future study, or part of a future study, is conducted on a smaller scale. We suggest that to facilitate their identification, these studies should be clearly identified using the terms 'feasibility' or 'pilot' as appropriate. This should include feasibility studies that are largely qualitative; we found these difficult to identify in electronic searches because researchers rarely used the term 'feasibility' in the title or abstract of such studies. Investigators should also report appropriate objectives and methods related to feasibility; and give clear confirmation that their study is in preparation for a future randomised controlled trial designed to assess the effect of an intervention.

Conflict of interest statement

Competing Interests: All authors have completed the ICMJE disclosure form at and declare support from the following organisations that might have an interest in the submitted work – Queen Mary University of London, Sheffield University, NIHR, Chief Scientist Office Scotland; financial relationships with NIHR, MRC, EC FP7, Canadian Institute for Health Research, Wiley, who might have an interest in the submitted work in the previous three years. No other relationships or activities have influenced the submitted work. This does not alter the authors' adherence to PLOS ONE policies on sharing data and materials.


Fig 1
Fig 1. NIHR definitions [5, 6].
Fig 2
Fig 2. Definitions of pilot and feasibility studies used in on-line Delphi survey.
Fig 3
Fig 3. Examples of different types of pilot and feasibility study used in the on-line Delphi survey [10, 11, 12].
Fig 4
Fig 4. Quotes from the on-line Delphi survey.
Fig 5
Fig 5. Four propositions presented at Edinburgh open meeting.
Fig 6
Fig 6. Flow chart showing identification of empirical pilot and feasibility studies.
Fig 7
Fig 7. Conceptual framework.

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Grant support

The authors received small grants from Queen Mary University of London (£7495), University of Sheffield (£8000), NIHR RDS London (£2000), NIHR RDS South East (£2400), Chief Scientist Office Scotland (£1000). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.