Clinical Review Understanding controlled trials

Randomisation methods in controlled trials

BMJ 1998; 317 doi: http://dx.doi.org/10.1136/bmj.317.7168.1301 (Published 07 November 1998) Cite this as: BMJ 1998;317:1301
  1. Chris Robertsa, senior research fellow,
  2. David Torgersonb, senior research fellow
  1. aNational Primary Care Research and Development Centre, University of Manchester, Manchester M13 6PL
  2. bNational Primary Care Research and Development Centre, Centre for Health Economics, University of York, York YO1 5DD

    The main purpose of randomisation is to avoid bias by distributing the characteristics of patients that may influence outcome randomly between treatment groups so that any difference in outcome can be explained only by treatment. These characteristics might be demographic ones such age or prognostic factors such as clinical history or disease severity. For example menopausal status may influence outcome of treatment for breast cancer.

    The most elementary form of randomisation is, in the case of two treatments, equivalent to allocating treatment by tossing a coin. Lists for allocating patients by simple randomisation may be constructed with tables of random numbers or random functions on pocket calculators or statistical software. Treatments may then be allocated to patients in sequence using numbered opaque envelopes containing treatment allocations or remotely by phone.

    While such simple randomisation will on …

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