It is highly desirable that during the conduct of a clinical trial, study subjects and investigators, both of whom are in a position to influence reported benefits and adverse effects, are unaware of the allocated treatment. A well-executed double-blind design protects against "ascertainment biases' by blinding both parties to the assigned treatment. The terms blinding and masking are used interchangeably.
The potential problem is that investigators and study subjects involved in clinical studies often have pre-conceived hopes and expectations regarding the study outcome. This could influence the trial findings, since most methods to assess benefits and harm rely to some extent on subjective judgment. Thus, knowledge of group assignment may consciously or unconsciously influence the evaluation of treatment effects. This has been reported many times in the literature.
A classic illustration is a placebo-controlled vitamin C trial for the prevention and treatment of the common cold that was conducted among employees at the National Institutes of Health.4 Many of the enrollees could not resist the temptation to analyze the content of their blinded study medications. Among the participants who did not break the blind, the mean duration of colds was similar in the two groups. In contrast, participants who knew they were taking vitamin C reported shorter cold durations than those who knew they took placebo!
Blinding is another major advantage of clinical trials compared to observational studies. Types and methods of blinding are discussed in Chapter 10.
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