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Table 1 Four main classes of randomisation procedure

From: Use of randomisation in clinical trials: a survey of UK practice

Class

Procedure

Simple randomisation

Each patient has a known chance, usually equal, of being given each treatment and the treatment to be given cannot be predicted in advance. The simplest method is tossing a coin, but one can also use random tables or a random number generator on a calculator or computer.

Restricted randomisation

Imposing specific constraints on the randomisation process (for example, random permuted blocks) with a view to ensuring balance in the number of patients allocated to each treatment.

Covariate-balancing randomisation

Often it is desirable not only to achieve similar numbers of patients in each treatment group but also to ensure that patient groups are similar with respect to important prognostic factors such as age or gender. A number of mechanisms have been put forward to ensure balance across important prognostic factors – the most common of these include stratification and minimisation.

Response-adaptive randomisation

The treatment assignments depend upon previous patient responses to treatment.