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Assessing allocation concealment and blinding in randomised controlled trials: why bother?

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The EBN users' guide in the previous issue of Evidence-Based Nursing outlined the primary and secondary questions for evaluating studies of healthcare interventions. One of the primary questions for assessing the validity of a study's findings is whether the assignment of patients to treatments was randomised and whether randomisation was concealed. One of the secondary questions is whether patients, clinicians, outcome assessors, and data analysts were unaware of (blinded to or masked from) patient allocation. Beginning with the October 1999 issue of Evidence-Based Nursing, allocation concealment and blinding have been given more attention. The “design” section of abstracts of randomised trials now includes a statement of whether randomisation was concealed from those responsible for entering patients into trials, and who was blinded to treatment allocation during the trials. These additional specifications provide readers with more information to judge the internal validity of trials. In this editorial, the background and rationale for these decisions are addressed.

Allocation concealment

Random allocation to intervention groups remains the only method of ensuring that the groups being compared are on an equivalent footing at study outset, thus eliminating selection and confounding biases. This has allowed randomised controlled trials (RCTs) to play a key part in advancing healthcare practice.

The success of randomisation depends on 2 interrelated processes.1, 2 The first entails generating a sequence by which the participants in a trial are allocated to intervention groups. To ensure unpredictability of that allocation sequence, investigators should generate it by a random process (eg, computer generated numbers, random number tables, or coin flipping). The second process, allocation concealment, shields those involved in a trial from knowing upcoming assignments in advance.3, 4 Without this protection, investigators have been known to change who gets the next assignment, making the comparison groups less equivalent.5, …

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