Double blind procedure is a method of enhancing internal validity in an experiment. In double blind procedure, neither the researcher nor the subjects are made aware of which group is the experimental group and which the control group.

In double blind procedure, experimental group in a clinical study is the group which receives the actual drug or treatment being studied. Controls, double blind procedure, and randomized testing are used to reduce error, self-deception and bias.

Control groups are used in controlled experiments to curb bias. The control group and experimental group must be identical in all relevant ways except for the introduction of a suspected causal agent into the experimental group.

In a double-blind procedure in clinical trials: two identical groups of patients are compared, one of which receives the drug and one of which receives a placebo. Neither the patients nor the doctor know which group receives the real drug, which serves to curb bias. Double blind procedure prevents the researcher from communicating expectations or the subjects acting in ways they think to be expected of them.

The double blind procedure is part of the scientific method that is used to prevent research outcomes from being ‘influenced’ by the placebo effect or observer bias. Double blinded research is used in many fields of research, including medicine, psychology, social sciences and forensic research. For example, in blind taste tests, where the brand identities are concealed, consumers may favor a different brand.

Many journals use stringent double-blind procedure, in which neither referees nor author(s) are informed of each other’s identity. Double-blind review is based on the principle that criticism is more impartial when authors’ identities are unknown to referees, who might be swayed in either positive or negative directions by authors’ reputations, personality traits, etc. A similar process is normally applied to grant proposals (Kassirer and Campion 1994). Few journals in the atmospheric sciences, for example, use double-blind review. But, many scientific communities are small enough that even double-blind referees and authors can often guess each other’s identity.

While double-blind review may not prevent authors and referees from guessing each other’s identities in a small field (and wrong guesses can turn out to be even more harmful than right ones).

In a single blind method, 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 or where the experimenter will not introduce further bias. However, there is a risk that subjects are influenced by interaction with the researchers – known as the experimenter’s bias. Single-blind trials are particularly risky in psychology and social science research, where the experimenter has an expectation of what the outcome should be, and may consciously or unconsciously influence the behavior of the subject.

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