EBMH Notebook

Assessing allocation concealment and blinding in randomised controlled trials: why bother? The scientific community’s quest for unbiased research received a strong boost from a recent policy amendment on randomised controlled trials (RCTs) in this journal. Henceforth, the status of allocation concealment will be clearly indicated in the abstracts along with that of blinding. Thus, readers will have additional information by which to judge the internal validity of trials. In this editorial I address the background and rationale for these enhancements. Background Random allocation to intervention groups remains the only method of ensuring that the groups being compared are on an equivalent footing at the outset of the study, thus eliminating selection and confounding biases. This has allowed RCTs to play a key part in advancing medical science. 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. The second process, allocation concealment, shields those involved in a trial from knowing upcoming assignments in advance.3 4 Without this protection, investigators and patients have been known to change who gets the next assignment, making the comparison groups less equivalent.5 6 For example, suppose that an investigator creates an adequate allocation sequence using a random number table. However, the investigator then affixes the list of that sequence to a bulletin board, with no allocation concealment. Those responsible for admitting participants could ascertain the upcoming treatment allocations and then route participants with better prognoses to the experimental group and those with poorer prognoses to the control group, or vice versa. Bias would result. Inadequate allo› cation concealment also exists, for example, when assignment to groups depends on whether a participant’s hospital number is odd/even, or depends on translucent envelopes that allow discernment of assignments when held to a light bulb. Allocation concealment should not be confused with blinding. Allocation concealment concentrates on preventing selection and confounding biases, safeguards the assignment sequence before and until allocation, and can always b