What is meant by intention to treat analysis? Survey of published randomised controlled trials
BMJ 1999; 319 doi: https://doi.org/10.1136/bmj.319.7211.670 (Published 11 September 1999) Cite this as: BMJ 1999;319:670
- a Medical Statistics Unit, Fylde College, Lancaster University, Lancaster LA1 4YF
- b AstraZeneca, Mereside, Alderley Park, Macclesfield, Cheshire SK10 4TG
- Correspondence to: S Hollis
- Accepted 2 June 1999
Abstract
Objectives: To assess the methodological quality of intention to treat analysis as reported in randomised controlled trials in four large medical journals.
Design: Survey of all reports of randomised controlled trials published in 1997 in the BMJ, Lancet, JAMA, and New England Journal of Medicine.
Main outcome measures: Methods of dealing with deviations from random allocation and missing data.
Results: 119 (48%) of the reports mentioned intention to treat analysis. Of these, 12 excluded any patients who did not start the allocated intervention and three did not analyse all randomised subjects as allocated. Five reports explicitly stated that there were no deviations from random allocation. The remaining 99 reports seemed to analyse according to random allocation, but only 34 of these explicitly stated this. 89 (75%) trials had some missing data on the primary outcome variable. The methods used to deal with this were generally inadequate, potentially leading to a biased treatment effect. 29 (24%) trials had more than 10% of responses missing for the primary outcome, the methodsof handling the missing responses were similar in this subset.
Conclusions: The intention to treat approach is often inadequately described and inadequately applied. Authors should explicitly describe the handling of deviations from randomised allocation and missing responses and discuss the potential effect of any missing response. Readers should critically assess the validity of reported intention to treat analyses.
Key messages
Intention to treat gives a pragmatic estimate of the benefit of a change in treatment policy rather than of potential benefit in patients who receive treatment exactly as planned
Full application of intention to treat is possible only when complete outcome data are available for all randomised subjects
About half of all published reports of randomised controlled trials stated that intention to treat was used, but handling of deviations from randomised allocation varied widely
Many trials had some missing data on the primary outcome variable, and methods used to deal with this were generally inadequate, potentially leading to bias
Intention to treat analyses are often inadequately described and inadequately applied
Footnotes
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Funding None.
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Competing interests None declared.
- Accepted 2 June 1999