Randomization. Part 3: allocation concealment and randomization implementation.

Randomization consists of 3 steps: generation of the random allocation sequence, allocation concealment, and implementation of the random allocation sequence. In the previous 2 articles, we discussed the methods to generate the random allocation sequence (the Table summarizes these methods); in this final article on randomization, we will discuss allocation concealment and implementation of randomization. Allocation concealment is the means to guarantee that the generated randomization lists and consequently the treatment allocations of the trial participants cannot be known or predicted by all involved persons. These include patients, investigators, and other personnel engaged in the study. The objective of allocation concealment is to reduce selection bias, and it can always be applied. Knowing or being able to predict the next treatment allocation might lead to subversion of the randomization in a way that validates the preconceptions of the investigators. For example, if we are conducting a study to compare time to alignment between conventional and self-ligating appliances, lack of allocation concealment could permit a biased investigator to undermine randomization and assign patients with more crowding to the group that he or she does not favor. In this manner, the ensuing selection bias will falsely overestimate or underestimate the effect of appliance type on the alleviation of crowding. In other words, if the investigator favors conventional appliances, he or she might randomize patients with more severe crowding to the selfligating group, thus making the conventional arm appear to reach alignment in a shorter time. Studies on allocation concealment have shown that lack of allocation concealment has been associated with greater and biased treatment effects. 1