Statistics Notes: Units of analysis
BMJ 1997; 314 doi: https://doi.org/10.1136/bmj.314.7098.1874 (Published 28 June 1997) Cite this as: BMJ 1997;314:1874
- Douglas G Altman, heada,
- Martin Bland J, professor of medical statisticsb
- a ICRF Medical Statistics Group, Centre for Statistics in Medicine, Institute of Health Sciences, Oxford OX3 7LF
- b Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE
- Correspondence to: Mr Altman
Article
In clinical studies the focus of interest is almost always the patient. If we carry out a randomised trial to compare two treatments we are interested in comparing the outcomes of patients who received each of the treatments. In some conditions several measurements will be taken on the same patient, but the focus of interest remains the patient. Failure to recognise this fact results in multiple counting of individual patients and can seriously distort the results. We explain this error below. Its frequency in medical research is indicated by the whole chapter devoted to it in Andersen's classic compilation.1
The simplest case is when researchers study a part of the human anatomy which is, so to speak, in duplicate: eyes, ears, arms, etc. At the other extreme very many measurements can be taken on a single patient. Such data arise frequently in dentistry, with measurements made on each tooth, or even each face of each tooth, and in rheumatology, …