8.9.2.2  Inadequate methods of sequence generation

Systematic methods, such as alternation, assignment based on date of birth, case record number and date of presentation are sometimes referred to as ‘quasi-random’. Alternation (or rotation, for more than two intervention groups) might in principle result in similar groups, but many other systematic methods of sequence generation may not. For example, the day on which a patient is admitted to hospital is not solely a matter of chance.

 

An important weakness with all systematic methods is that concealing the allocation schedule is usually impossible, which allows foreknowledge of intervention assignment among those recruiting participants to the study, and biased allocations (see Section 8.10).

 

Example (of high risk of bias): We allocated patients to the intervention group based on the week of the month.

 

Example (of high risk of bias): Patients born on even days were assigned to Treatment A and patients born on odd days were assigned to Treatment B.