![]() ![]() Risk of bias may be judged in the same way whether or not a trial claims to have stratified.Īnother approach that incorporates both the general concepts of stratification and restricted randomization is minimization, which can be used to make small groups closely similar with respect to several characteristics. If simple (rather than restricted) randomization was used in each stratum, then stratification would have no effect but the randomization would still be valid. This generates separate randomization schedules for subsets of participants defined by potentially important prognostic factors, such as disease severity and study centres. We used a computer random number generator to select random permuted blocks with a block size of eight and an equal allocation ratio.Īlso common is stratified randomization, in which restricted randomization is performed separately within strata. The block size may be randomly varied to reduce the likelihood of foreknowledge of intervention assignment.Įxample (of low risk of bias): We used blocked randomization to form the allocation list for the two comparison groups. Blocking ensures that the numbers of participants to be assigned to each of the comparison groups will be balanced within blocks of, for example, five in one group and five in the other for every 10 consecutively entered participants. Blocked randomization (random permuted blocks) is a common form of restricted randomization (Schulz 2002c, Schulz 2006). Sometimes restricted randomization is used to generate a sequence to ensure particular allocation ratios to the intervention groups (e.g. ‘case-mix’ variation) (Altman 1999).Įxample (of low risk of bias): We generated the two comparison groups using simple randomization, with an equal allocation ratio, by referring to a table of random numbers. In trials using small samples, simple randomization will sometimes result in an allocation sequence leading to groups that differ, by chance, quite substantially in size or in the occurrence of prognostic factors (i.e. In trials using large samples (usually meaning at least 100 in each randomized group (Schulz 2002c, Schulz 2002d, Schulz 2006), simple randomization generates comparison groups of relatively similar sizes. More usually it is achieved by referring to a published list of random numbers, or to a list of random assignments generated by a computer. In principle, this could be achieved by allocating interventions using methods such as repeated coin-tossing, throwing dice or dealing previously shuffled cards (Schulz 2002c, Schulz 2006). Randomization with no constraints to generate an allocation sequence is called simple randomization or unrestricted randomization. The use of a random component should be sufficient for adequate sequence generation. 8.9.2.1 Adequate methods of sequence generation ![]() For the current version, please go to /handbook/current or search for this chapter here. This is an archived version of the Handbook. ![]()
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