Expectations influence results. Someone who knows they are receiving a treatment may feel better for that reason alone, and whoever assesses them may, without meaning to, interpret the treated group more favourably. Blinding removes that bias: if nobody knows who is who, expectations are spread equally.
The placebo is the tool that makes it possible: a preparation with the same appearance and form of administration as the treatment, but without its active ingredient. The “placebo effect” (the improvement that appears just from receiving something) can be substantial for subjective symptoms such as pain, mood or energy.
When a treatment has very obvious effects, keeping the blind is difficult, which must also be taken into account when reading a study.
Results from open-label (unblinded) studies, especially for subjective outcomes, tend to overestimate effects. It is one of the first things to check in a paper.
Related terms
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- Hierarchy of evidence and development phases · Module 13
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