Education And Debate Statistics notes

# How to randomise

BMJ 1999; 319 (Published 11 September 1999) Cite this as: BMJ 1999;319:703
1. Douglas G Altman, professor of statistics in medicinea,
2. J Martin Bland, professor of medical statisticsb
1. a ICRF Medical Statistics Group, Centre for Statistics in Medicine, Institute of Health Sciences, Oxford OX3 7LF
2. b Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE
1. Correspondence to: Professor Altman

We have explained why random allocation of treatments is a required feature of controlled trials.1 Here we consider how to generate a random allocation sequence.

Almost always patients enter a trial in sequence over a prolonged period. In the simplest procedure, simple randomisation, we determine each patient's treatment at random independently with no constraints. With equal allocation to two treatment groups this is equivalent to tossing a coin, although in practice coins are rarely used. Instead we use computer generated random numbers. Suitable tables can be found in most statistics textbooks. The table shows an example2: the numbers can be considered as either random digits from 0 to 9 or random integers from 0 to 99.

For equal allocation to two treatments we could take odd and even numbers to indicate treatments A and B respectively. We must then choose an arbitrary place to start and also the direction in which to read the table. The first 10 two digit numbers from a starting place in column 2 are 85 80 62 36 96 56 17 17 23 87, which translate into the sequence A B B B B B …

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