The concealment of group assignment (to either the treatment or control group) from the knowledge of participants and/or investigators in a clinical trial. Blinding eliminates the possibility that knowledge of assignment may affect individual response to treatment or investigator behaviours that may affect outcomes. Blinding is not always practical (e.g. when comparing surgery to drug treatment) but it should be used whenever it is possible and compatible with optimal care. There are various kinds of blinding:
single-blind trial: one in which knowledge of group assignment is withheld only from participants
double-blind trial: one in which the knowledge is withheld from participants and investigators
triple-blind trial: one in which the knowledge is withheld from participants, investigators and those assessing outcomes of the assignment.
Blinding has been defined 3 different ways in documents like National Institutes of Health Clinical Trial Glossary, Glossary of Key Information Security Terms, A Glossary of Terms for Community Health Care and Services for Older Persons.