Allocation concealment is a critical aspect of clinical trial design, particularly in randomized controlled trials (RCTs). It refers to the process of concealing the allocation sequence—meaning that researchers, participants, or both do not know which treatment group a participant will be assigned to until they are actually assigned. This helps to prevent selection bias, ensuring that the allocation of participants to different treatment groups is random and not influenced by either the researchers' or the participants' expectations or preferences.
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Allocation concealment prevents those deciding whether to enroll an eligible participant from knowing the upcoming randomized assignment. It protects the randomized controlled trial against selection based on that assignment. It concerns access to the assignment before enrollment; keeping participants or assessors unaware after assignment is a separate design feature.