This is an archived version of the Handbook. For the current version, please go to or search for this chapter here.  Adequate methods of allocation sequence concealment

Table 8.10.a provides minimal criteria for a judgement of adequate concealment of allocation sequence (left) and extended criteria which provide additional assurance that concealment of the allocation sequence was indeed adequate (right).


Examples (of low risk of bias) [published descriptions of concealment procedures judged to be adequate, as compiled by Schulz and Grimes (Schulz 2002b)]:

“ . . . that combined coded numbers with drug allocation. Each block of ten numbers was transmitted from the central office to a person who acted as the randomization authority in each centre. This individual (a pharmacist or a nurse not involved in care of the trial patients and independent of the site investigator) was responsible for allocation, preparation, and accounting of trial infusion. The trial infusion was prepared at a separate site, then taken to the bedside nurse every 24 h. The nurse infused it into the patient at the appropriate rate. The randomization schedule was thus concealed from all care providers, ward physicians, and other research personnel.” (Bellomo 2000).


“. . . concealed in sequentially numbered, sealed, opaque envelopes, and kept by the hospital pharmacist of the two centres.” (Smilde 2001).


“Treatments were centrally assigned on telephone verification of the correctness of inclusion criteria . . .” (de Gaetano 2001).


“Glenfield Hospital Pharmacy Department did the randomization, distributed the study agents, and held the trial codes, which were disclosed after the study.” (Brightling 2000).