Many good approaches to allocation concealment incorporate external involvement. Use of a pharmacy or central telephone randomization system are two common techniques. When external involvement is not feasible, an excellent allocation concealment mechanism is the use of numbered containers. Enclosing assignments in sequentially numbered, opaque, sealed envelopes can be a good allocation concealment mechanism if done with diligent development and monitoring. This method can be corrupted, however, particularly if poorly executed.
Recent studies provide empirical evidence of bias leaking into trials.
These results provide strong empirical evidence that inadequate allocation concealment contributes to bias in estimating treatment effects. Despite the importance of the mechanism of allocation, published reports frequently omit details.