Allocation bias

Systematic difference in how participants are assigned to comparison groups in a clinical trial.


Allocation bias may result if investigators know or predict which intervention the next eligible participant is supposed to receive. This knowledge may influence the way investigators approach potentially eligible participants and how they are assigned to the different groups, thereby selecting participants with good prognoses (i.e. anticipated good outcomes and treatment responses) into one group more than another.

In a trial of different blood pressure medications the use of sealed envelopes to conceal the allocation schedule resulted in imbalances in baseline blood pressure between the treatment and control groups. In turned out that participants in the control group already had lower blood pressures compared to participants in the treatment group at the outset. The observed imbalance could have arisen if the investigator opened the envelopes before allocating participants to groups.


Extract of the paper

In the nine trials using good allocation concealment, there was no detected effect of hypothermia treatment on death, whereas in the trials that did not conceal allocation there was a 38% reduction in the risk of death. Hypothermia for traumatic head injury. Cochrane Database of Systematic Reviews 2009.


There is evidence that over 80% of trials have unclear allocation concealment. Trials in which allocation was inadequately concealed reported estimates that were between 7% and 40% larger than effects in trials in which allocation was adequately concealed, although the size and direction of the effect were not predictable. A simulation of trials showed that, due to knowledge of previous allocations, as many as 1 in 5 trials could conclude that there was a difference when in fact there was not.

Snippet of an article about allocation concealment

Trials with inadequate allocation concealment yield estimates that are up to 40% larger than those that do not. Empirical evidence of bias. JAMA. 1995.

Preventive steps

Some standard methods of ensuring concealment of group allocation prior to allocation include sequentially numbered, opaque, sealed envelopes (SNOSE); sequentially numbered containers; pharmacy controlled allocation; and central allocation.

When trialists do more to show that group allocation (prior to allocation) was concealed, this permits more confidence that allocation bias has not affected the study’s results. For example, an audit trail can be recorded for sequentially numbered envelopes; sequentially numbered containers should look identical to each other from the outside; study staff should have correct training for maintaining allocation concealment; the block sizes for randomization should not be known; and all these features of the trial should be reported in the trial publications.

Where cluster randomisation or minimisation are used in the allocation to treatment or control groups, it may not be possible to prevent future allocations being predictable and thus concealment might not be adequate. In such cases, pre-defined statistical adjustments (e.g. covariate analysis) can be considered and should be reported in trial publications.

Researchers should try to understand the reasons for allocation concealment and the problems that can arise if a robust trial protocol including allocation concealment is not followed. Allocation concealment procedures should also be specified in a trial’s protocol, and reported in detail in any publication of the trial’s results.


Berger VW et al. Allocation concealment continues to be misunderstood. J Clin Epidemiol 2010; 63 (4): 468–470

Berger VW. Risk of selection bias in randomized trials: further insight. Trials. 2016 Oct 7;17(1):485.

Chalmers TC et al. Bias in treatment assignment in controlled clinical trials. N Engl J Med. 1983;309:1358-136

Odgaard-Jensen J et al. Randomisation to protect against selection bias in healthcare trials. Cochrane Database Syst Rev 2011;(4):MR000012. doi: 10.1002/14651858.MR000012.pub3

Page MJ et al. Empirical evidence of study design biases in randomized trials: systematic review of meta-epidemiological studies. PLoS One. 2016 Jul 11;11(7):e0159267. doi: 10.1371/journal.pone.0159267. eCollection 2016.

Pocock SJ et al. Sequential treatment assignment with balancing for prognostic factors in the controlled clinical trial. Biometrics. 1975 Mar;31(1):103-15.

Pildal J et al. Comparison of descriptions of allocation concealment in trial protocols and the published reports: cohort study. BMJ. 2005 May 7;330(7499):1049.

Savović J et al. Influence of reported study design characteristics on intervention effect estimates from randomized, controlled trials. Ann Intern Med. 2012 Sep 18;157(6):429-38.

Schulz KF  et al. Empirical evidence of bias. Dimensions of methodological quality associated with estimates of treatment effects in controlled trials. JAMA. 1995 Feb 1;273(5):408-12

Tamm M et al. Influence of selection bias on the test decision. A simulation study. Methods Inf Med. 2012;51(2):138-43. doi: 10.3414/ME11-01-0043. Epub 2011 Nov 21. Erratum in: Methods Inf Med. 2014;53(5):343

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