Location of Repository

Design, analysis and presentation of factorial randomised controlled trials

By Alan A Montgomery, Tim J Peters and Paul Little


Background<br/>The evaluation of more than one intervention in the same randomised controlled trial can be achieved using a parallel group design. However this requires increased sample size and can be inefficient, especially if there is also interest in considering combinations of the interventions. An alternative may be a factorial trial, where for two interventions participants are allocated to receive neither intervention, one or the other, or both. Factorial trials require special considerations, however, particularly at the design and analysis stages.<br/>Discussion<br/>Using a 2 × 2 factorial trial as an example, we present a number of issues that should be considered when planning a factorial trial. The main design issue is that of sample size. Factorial trials are most often powered to detect the main effects of interventions, since adequate power to detect plausible interactions requires greatly increased sample sizes. The main analytical issues relate to the investigation of main effects and the interaction between the interventions in appropriate regression models. Presentation of results should reflect the analytical strategy with an emphasis on the principal research questions. We also give an example of how baseline and follow-up data should be presented. Lastly, we discuss the implications of the design, analytical and presentational issues covered.<br/>Summary<br/>Difficulties in interpreting the results of factorial trials if an influential interaction is observed is the cost of the potential for efficient, simultaneous consideration of two or more interventions. Factorial trials can in principle be designed to have adequate power to detect realistic interactions, and in any case they are the only design that allows such effects to be investigated

Topics: R1
Year: 2003
OAI identifier: oai:eprints.soton.ac.uk:8192
Provided by: e-Prints Soton

Suggested articles



  1. (2001). Altman DG: The CONSORT statement: revised recommendations for improving the quality of reports of parallel-group randomised trials. Lancet doi
  2. (1999). Effect of antidepressant drug counselling and information leaflets on adherence to drug treatment in primary care: randomised controlled trial. BMJ doi
  3. (2001). G: Subgroup analyses in randomised controlled trials quantifying the risks of false-positives and false-negatives. Health Technol Assess doi
  4. (2001). Guidelines and educational outreach visits from community pharmacists to improve prescribing in general practice: a randomised controlled trial. doi
  5. (2001). Improving attendance for breast screening among recent non-attenders: a randomised controlled trial of two interventions in primary care. doi
  6. (1992). Interpretation of interaction in factorial analysis of variance design – letter. Statistics in Medicine
  7. (1991). Interpretation of interaction in factorial analysis of variance design. Statistics in Medicine doi
  8. (2003). JAC: Medical statistics Secondth edition.
  9. (2002). JNS: Statistical methods in medical research Fourthth edition. doi
  10. (2002). KF: An overview of clinical research: the lay of the land. Lancet doi
  11. (2003). Modelling binary data Secondth edition. Boca
  12. (2002). Randomised controlled trials in primary care: scope and application.
  13. (1997). Statistical issues in drug development Chichester: doi
  14. (2003). TJ: A factorial randomised controlled trial of decision analysis and an information video plus leaflet for newly diagnosed hypertensive patients.

To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.