for: double bind

The blind method is a part of the scientific method, used to prevent research outcomes from being influenced by either the placebo effect or the observer bias. To blind a person involved in research (whether a researcher, subject, funder, or other person) is to prevent them from knowing certain information about the process. The terms 'blind' (adj) or 'to blind' (vt) when used in this sense are figurative extensions of the literal idea of blindfolding someone. Blinded research is an important tool in many fields of research, from medicine, to psychology and the social sciences, to forensics.

There can be varying degrees of blinding (eg, single-blind, double-blind, triple-blind), as explained more below.

The opposite of a blind trial is an open trial.

Blinding is a basic tool to prevent conscious as well as subconscious bias in research. For example, in open taste tests comparing different product brands, consumers usually choose their regular brand. However, in blind taste tests, where the brand identities are concealed, consumers may favor a different brand.

Occasionally researchers prefer to use the terms masked (adj) or to mask (vt) instead of 'blind/blinded/to blind'. This preference is operative especially in ophthalmology, where the same word, 'blind', also frequently occurs in its literal sense of blindness. In these contexts it is simply more convenient to avoid any ambiguity, or annoyance to the reader, from the same word being used in two senses in close proximity. Nevertheless, in most research contexts, there is no expectation that 'blind/blinded' will be obsessively avoided via replacement with 'mask/masked'.

Single-blind trials

Single-blind describes experiments where information that could introduce bias or otherwise skew the result is withheld from the participants, but the experimenter will be in full possession of the facts.

In a single-blind experiment, the individual subjects do not know whether they are so-called "test" subjects or members of an "experimental control" group. Single-blind experimental design is used where the experimenters either must know the full facts (for example, when comparing sham to real surgery) and so the experimenters cannot themselves be blind, or where the experimenters will not introduce further bias and so the experimenters need not be blind. However, there is a risk that subjects are influenced by interaction with the researchers — known as the experimenter's bias. Single-blind trials are especially risky in psychology and social science research, where the experimenter has an expectation of what the outcome should be, and may consciously or subconsciously influence the behavior of the subject.

Double-blind trials

Double-blind describes an especially stringent way of conducting an experiment, usually on human subjects, in an attempt to eliminate subjective bias on the part of both experimental subjects and the experimenters. In most cases, double-blind experiments are held to achieve a higher standard of scientific rigour.