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— An inert substance, device or procedure used as a comparator in studies assessing the effects of a treatment

Full explanation:

A placebo is a dummy or sham treatment that does not contain active ingredients, which has been designed to be indistinguishable from the active treatment(s) being assessed.

It is used to blind participants and others involved in a study of treatment effects, and to reduce the risk of placebo effects; i.e. effects that are, or could be caused by an inactive treatment, presumed to act psychologically through suggestion.

Placebos can help prevent differences in the care provided to the participants in a study, other than the treatments being compared (performance bias), and so reduce differences in how outcomes are measured in treatment comparison groups (measurement bias).

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