What is a Blinded and Unblinded Clinical Trial?

A blinded clinical trial is one where participants do not know which treatment or medical intervention they have been allocated. In a blind clinical trial, certain information which may influence the participants in the trial (including subjects, CRAs, and evaluators) is withheld or hidden (blinded) until after the trial is complete. Good blinding can reduce or eliminate experimental biases that arise from participant expectation, observer bias, confirmation bias, and so on.

An unblinded clinical trial is one where information is not withheld from trial participants and, in such cases, both participants and researchers know which treatment is being administered. During the course of a clinical trial, a participant becomes unblinded if they obtain information that has been withheld or hidden (blinded) from them. However, if this occurs unintentionally before the end of the trial, then this can be a source of experimental error.