This is an archived version. For the current version, please go to

Table 8.10.a: Minimal and extended criteria for judging concealment of allocation sequence to be adequate (low risk of bias)

Minimal criteria for a judgement of adequate concealment of the allocation sequence

Extended criteria providing additional assurance

Central randomization.

The central randomization office was remote from patient recruitment centres. Participant details were provided, for example, by phone, fax or email and the allocation sequence was concealed to individuals staffing the randomization office until a participant was irreversibly registered.

Sequentially numbered drug containers.

Drug containers prepared by an independent pharmacy were sequentially numbered and opened sequentially. Containers were of identical appearance, tamper-proof and equal in weight.

Sequentially numbered, opaque, sealed envelopes.

Envelopes were sequentially numbered and opened sequentially only after participant details were written on the envelope. Pressure sensitive or carbon paper inside the envelope transferred the participant’s details to the assignment card. Cardboard or aluminium foil inside the envelope rendered the envelope impermeable to intense light. Envelopes were sealed using tamper-proof security tape.