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For the simple situation in which the attending clinician is also the assessor of the outcome, the trial should ideally be double‐blind (or double‐masked). This means that neither the patient nor the attending clinician will know the actual treatment allocated. Having no knowledge of which treatment has been taken, neither the patient nor the clinician can be influenced at the assessment stage by such knowledge. In this way, an unprejudiced evaluation of the patient response is obtained. Thus Meggitt, Gray and Reynolds (2006) used double‐blind formulations of Azathioprine or Placebo so that neither the patients with moderate‐to‐severe eczema, nor their attending clinical team, were aware of who received which treatment. Although they did not give details, the blinding is best broken only at the analysis stage once all the data had been collated.

Despite the inherent advantage of this double‐blind design, most clinical trials cannot be conducted in this way as, for example, a means has to be found for delivering the treatment options in an identical way. This may be a possibility if the standard and test are available in tablet form of identical colour, shape, texture, smell and taste. If such ‘identity’ cannot be achieved, then a single‐blind design may ensue. In such a design, one of the patient or the clinical assessor has knowledge of the treatment being given but the other does not. In trials with patient survival time as the endpoint, double‐blind usually means that both the patient and the treating physician and other staff are blinded. However, assessment is objective (death) and the blinding irrelevant by this stage.

Finally, and this is possibly the majority situation, there will circumstances in which neither the patient nor the assessor can be blind to the treatments actually received. Such designs are referred to as ‘open’ or ‘open‐label’ trials.