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Randomisation (or random allocation)


Method analogous to tossing a coin to assign patients to treatment groups (the experimental treatment is assigned if the coin lands heads and a conventional, control or placebo treatment is given if the coin lands tails). Usually done by using a computerthat generates a list of random numbers, which can then be used to generate a treatment allocation list.

Randomised controlled clinical trial (RCT)

A group of patients is randomised into an experimental group and a control group. These groups are followed up for the variables/outcomes of interest. The point about using randomisation is that it avoids any possibility of selection bias in a trial. The test that randomisation has been successful is that different treatment groups have same characteristics at baseline. For instance, there should be the same number of men and women, or older or younger people, or different degrees of disease severity.

Quasi-random allocation

A method of allocating participants to different forms of care that is not truly random; for example, allocation by date of birth, day of the week, medical record number, month of the year, or the order in which participants are included in the study (alternation).

A quasi-randomised trial uses quasi-random method of allocating participants to different interventions. There is a greater risk of selection bias in quasi-random trials where allocation is not adequately concealed compared with randomised controlled trials with adequate allocation concealment.

Stratified randomisation

In any randomised trial it is desirable that the comparison groups should be as similar as possible as regards those characteristics that might influence the response to the intervention. Stratified randomisation is used to ensure that equal numbers of participants with a characteristic thought to affect prognosis or response to the intervention will be allocated to each comparison group. For example, in a trial of women with breast cancer, it may be important to have similar numbers of pre-menopausal and post-menopausal women in each comparison group. Stratified randomisation could be used to allocate equal numbers of pre- and post-menopausal women to each treatment group. Stratified randomisation is performed either by performing separate randomisation (often using random permuted blocks) for each strata, or by using minimisation.