Introduction to pilot and feasibility studies

There are an increasing number of publications of studies referred to as pilot and feasibility studies. However, there are sometimes differences of opinion as to what these terms mean.

What are pilot and feasibility studies?

A recently developed framework suggests the following definitions:

  • A feasibility study asks whether something can be done, should we proceed with it, and if so, how.

  • A pilot study asks the same question but has a specific design feature: in a pilot study a future study, or part of a future study, is conducted on a smaller scale.

The corollary of these definitions is that all pilot studies are feasibility studies but not all feasibility studies are pilot studies. This framework is based on extensive research into the use and opinion of these terms by the research community and more general definitions of the terms:

Internal/external pilot studies

Avery et al define internal and external pilot studies as follows:

  • An external pilot is a rehearsal of the main study where the outcome data are not included as part of the main trial outcome data set.

  • In an internal pilot, the pilot phase forms the first part of the trial and the outcome data generated may contribute to the final analysis.

The choice of design (internal vs. external) should be pre-specified to avoid bias. External pilot studies allow for greater flexibility to change the trial design for the main study once the pilot is complete, whereas internal pilot studies may be seen as more efficient. For more information on this increase in efficiency, see the following paper:

Definitions of pilot and feasibility studies by funders

The definitions in the framework above have been adopted by the Health Research Board in Ireland to guide researchers applying for funding.

However, other funding bodies currently use different definitions. For example, the UK National Institute for Health Research uses mutually exclusive definitions of pilot and feasibility studies.

The Medical Research Council (MRC) puts pilot and feasibility studies together under one umbrella.

Much of the focus on pilot and feasibility studies is related to such studies carried out in advance of a randomised controlled trial. Both the framework and the NIHR definitions have this focus. We recommend the definitions from the framework because of their simplicity and grounding in robust research.

Incorrect use of the terms pilot and feasibility

Some recent reviews, as listed below, highlighted that sometimes a small underpowered effectiveness study is labelled as a pilot or feasibility study. There is therefore a need to raise awareness of the difference between a pilot study which is designed to clarify areas of uncertainty, and a small underpowered study labelled as a pilot which does not comply with the CONSORT guidance:

Examples of pilot and feasibility studies

Below are some examples of well-conducted and reported pilot and feasibility studies:

Why do a pilot or feasibility study?

The purpose of performing pilot and feasibility studies is to give your research the best chance of success. When developing an intervention, the MRC suggest that a series of pilot or feasibility studies may be needed to target each of the key uncertainties in the design, before moving on to a definitive evaluation.

A pilot or feasibilty study in advance of a future definitive study is helpful to investigate areas of uncertainty about the later definitive study, for example, checking participants can be recruited and retained, testing the study protocol, and exploring potential outcome measures. Assessing feasibility in this way ensures that fewer problems are likely to be encountered in the definitive study. This encourages methodological rigour and higher quality main studies.

From a practical point of view, funders often require pilot or feasibility studies to determine whether a definitive study is likely to succeed and hence be worth funding. Moreover, funders often expect applicants to use data from the pilot or feasibility study to plan study duration, number of sites etc.

For further reading on why to do a pilot or feasibility study:

Types of design

All sorts of designs can be used depending on the aims of the pilot or feasibility study, including qualitative and quantitative.

For information on how to choose the design, see Resources – design.