Statistics notes: MatchingBMJ 1994; 309 doi: https://doi.org/10.1136/bmj.309.6962.1128 (Published 29 October 1994) Cite this as: BMJ 1994;309:1128
- J M Bland,
- D G Altman
- Correspondence to: Mr Bland
In many medical studies a group of cases, people with a disease under investigation, are compared with a group of controls, people who do not have the disease but who are thought to be comparable in other respects. This happens in epidemiological case-control studies, where a possible risk factor is compared between cases and controls to investigate the nature of the disease. In both types of study cases and controls are sometimes matches. This means that for every case there is a control who has the same (or closely similar) values of the matching variables. Matching may be by sex, age to within five years, ethnic group, etc. Sometimes there are two or more such controls for each case.
We match to ensure that controls and cases …