Randomised controlled trials are the best research design for decisions about the effect of different interventions but randomisation does not, of itself, promote the applicability (sometimes called 'generalisability') of a trial's results to situations other than the precise one in which the trial was done. Unfortunately, it is very common that an intervention that has been shown to be effective in a trial context never becomes part of routine healthcare practice because, for one reason or another, it is not easily applicable to the 'real world'. While methodologists and trialists have rightly paid great attention to internal validity, much less has generally been given to applicability (1).

The Normalisation Process Theory (NPT) may be able to help because it offers a theoretical framework with which to optimise the development a trial intervention. There are three main ways in which the theory could do this:

  • to support intervention design
  • to describe the context of a trial
  • to support the interpretation of a trial's results

Before discussing these three points further it is important to note that NPT could also be used to optimise the trial parameters by asking eg. ‘To what extent do proposed trial procedures 'fit' with existing and routine ways of working?’ or ‘Do trial procedures become routine during the course of the trial?’ Addressing these questions might be a way of improving the applicability of the intervention from the trial context to the 'real world' to which we now return.

Finally, it is important to remember that NPT is neither a protocol about how to do a trial nor a recipe for how to understand it. It is designed to sensitise thinking about certain aspects of trials. NPT can be used alongside other methods, tools and methodologies to support your trial. It is intended to be used in a flexible manner depending on the specifics of a given trial.

Things to consider

  • Who are the people I expect to use the results of my trial and what can I do to make sure that these people will not be forced to dismiss my trial as irrelevant to them, their patients, or their healthcare systems?

References and further reading

  1. Treweek S, Zwarenstein M. Making trials matter: pragmatic and explanatory trials and the problem of applicability. Trials 2009; 10:37.
  2. Rothwell PM. Treating Individuals 1—External validity of randomised controlled trials: “To whom do the results of this trial apply?”Lancet 2005; 365: 82-93.
  3. Loudon K, Treweek S, Sullivan F, Donnan P, Thorpe KE, Zwarenstein M. The PRECIS-2 tool: designing trials that are fit for purpose. BMJ 2015; 350: h2147–7.