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The definition of concealed allocation: Did the investigators know to which group a potential subject would be assigned before they were actually enrolled in this study? Concealed allocation is used in randomized studies to protect the integrity of the randomization process. Randomization is meant to prevent selection bias, and if there was no concealed allocation, the study is again susceptible to selection bias. In this case, the selection bias is occurring during the enrollment period and causes the study population to be unrepresentative of the population of potential subjects.
Trials that do not use conceal allocation consistently overestimate the benefit of the treatment by 40%.‡ The recent changes in recommendations for breast cancer screening using mammography were prompted by a discovery almost 15 years ago that the lack of allocation concealment biased studies that evaluated the effectiveness of screening mammography.
Allocation concealment is not the same thing as blinding. Allocation concealment occurs before a study begins, during the process of selecting patients for a study. It is possible to have a study that is blinded, but does not conceal allocation. It’s also possible to have a non blinded study that does conceal allocation.
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