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8.10.2  Assessing risk of bias in relation to adequate or inadequate allocation sequence concealment

The following considerations may help review authors assess whether concealment of allocation was sufficient to protect against bias, when using the Collaboration’s tool (Section 8.5).


Proper allocation sequence concealment secures strict implementation of an allocation sequence without foreknowledge of intervention assignments. Methods for allocation concealment refer to techniques used to implement the sequence, not to generate it (Schulz 1995b). However, most allocation sequences that are deemed inadequate, such as allocation based on day of admission or case record number, cannot be adequately concealed, and so fail on both counts. It is theoretically possible, yet unlikely, that an inadequate sequence is adequately concealed (the person responsible for recruitment and assigned interventions would have to be unaware that the sequence being implemented was inappropriate). However, it is not uncommon for an adequate (i.e. randomized) allocation sequence to be inadequately concealed, for example if the sequence is posted on the staff room wall.


Some review authors confuse allocation concealment with blinding of allocated interventions. Allocation concealment seeks to prevent selection bias in intervention assignment by protecting the allocation sequence before and until assignment, and can always be successfully implemented regardless of the study topic (Schulz 1995b, Jüni 2001). In contrast, blinding seeks to prevent performance and detection bias by protecting the sequence after assignment (Jüni 2001, Schulz 2002a), and cannot always be implemented – for example, in trials comparing surgical with medical treatments. Thus, allocation concealment up to the point of assignment of the intervention and blinding after that point address different sources of bias and differ in their feasibility.


The importance of allocation concealment may depend on the extent to which potential participants in the study have different prognoses, whether strong beliefs exist among investigators and participants regarding the benefits or harms of assigned interventions, and whether uncertainty about the interventions is accepted by all people involved (Schulz 1995a). Among the different methods used to conceal allocation, central randomization by a third party is perhaps the most desirable. Methods using envelopes are more susceptible to manipulation than other approaches (Schulz 1995b). If investigators use envelopes, they should develop and monitor the allocation process to preserve concealment. In addition to use of sequentially numbered, opaque, sealed envelopes, they should ensure that the envelopes are opened sequentially, and only after the envelope has been irreversibly assigned to the participant.