Randomization without restriction. In a two-group trial, it is analogous to the toss of a coin.
Block randomisation allows the trial investigators to control the numbers of subjects allocated to each group during the enrollment phase of the study by using blocks of sequences of allocation assignment, wherein each letter represents an intervention or arm of the trial, (e.g. blocks of four might consist of: AABB, ABAB, ABBA, etc). This example shows an equal distribution of participants to each arm of the study but this might not always be the case.
The blocks can be of any size, but ideally the size should correspond to a multiple of the number of groups in the study. (Altman DG. Practical statistics for medical research. London: Chapman & Hall, 1991.)
Stratified block randomisation
Stratified block randomisation can further restrict chance imbalances to ensure the treatment groups are as alike as possible for selected prognostic variables or other patient factors. A set of permuted blocks is generated for each combination of prognostic factors. This ensures that treatments are balanced at the end of every strata block.
Minimisation is used as an alternative method to stratified randomisation to provide treatment groups that are similar for several variables. Stratified randomisation on several variables can lead to large numbers of strata. This can lead to a large number of incomplete blocks and to a substantial imbalance overall which minimisation can avoid.
The principle behind minimisation:
People are allocated (with a probability greater than 50%) to the treatment group that would minimise the imbalance between the groups with respect to the variables of interest.
Criteria for method:
This method depends on the characteristics of the person entering the trial (the first person is entered by simple randomisation) and therefore no pre-arranged list can be constructed.
Suppose that 20 people have been entered into the trial in this centre and that the next person that is ready to receive the treatment assignment is <10, male and <2 years since diagnosis. A table can be constructed as follows for a similar table/explanation:
|Time since diagnosis||< 2 Years |
2 Years +
The treatment allocation for this person is determined by looking at the number of people already in the study with these characteristics by treatment group, i.e.
Therefore to make the groups as similar as possible allocate this person (with probability > 1/2) to the placebo group.