Sociology Index


Experimental Group, Control Group, Randomized testing

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.

The Effects of Double-Blind versus Single-Blind Reviewing: Experimental Evidence from The American Economic Review 
Rebecca M. Blank - The American Economic Review, Vol. 81, No. 5 (Dec., 1991)
Abstract: The results from a randomized experiment conducted at The American Economic Review on the effects of double-blind versus single-blind peer reviewing on acceptance rates and referee ratings indicate that acceptance rates are lower and referees are more critical when the reviewer is unaware of the author's identity. These patterns are not significantly different between female and male authors. Authors at top-ranked universities and at colleges and low-ranked universities are largely unaffected by the different reviewing practices, but authors at near-top-ranked universities and at nonacademic institutions have lower acceptance rates under double-blind reviewing.

An Alternative to the Double Blind Procedure 
WILLIAM GUY PH.D., Program Coordinator, Biometric Laboratory, George Washington University.
MARTIN GROSS M.D., and HELEN DENNIS B.A., Research Department, Springfield State Hospital, Sykesville, Md. - American Psychiatric Association
Therapists' bias may influence the assessment of treatment results even when the procedure purports to be a blind one. The authors advocate the employment of an independent assessment team as an alternative to the classical double blind technique. They feel the IAT serves as an additional control over therapist bias and permits the use of modified double blind procedures in a variety of research situations.