Quantification methods were developed for selection bias by predictability of allocations with unequal block randomization.

BACKGROUND AND OBJECTIVE

A selection of patients for a controlled clinical trial may be biased because of prior knowledge of the treatment. With randomized blocks of known or guessed lengths, some allocations can be predicted with certainty. Previously described methods determine the proportion of predictable cases for blocks of equal lengths. It may be useful to make a calculation for unequal blocks as well to find a method that reduces this predictability.

STUDY DESIGN AND SETTING

Quantification methods are developed for series of two and three unequal blocks, using the probability of identifying a long block when it comes before a short one if it starts with a sequence incompatible with the content of a short block. Results are compared with the recently described maximal allocation procedure.

RESULTS

Predictability is not always reduced by unequal blocks and is even worse in some cases, compared to equal blocks. Predictability is not necessarily decreased with the maximal allocation procedure.

CONCLUSIONS

Before choosing an allocation method, it is important to quantify the predictability of possible options to reduce selection bias. Several practical recommendations are formulated for choosing methods, taking this risk of bias into account.