Double-Blind Design

Double-blind design refers to an experimental methodology with treatment and control groups where neither participants nor researchers, including investigators and outcome assessors alike, know who belongs to the treatment group and who belongs to the control group. This entry describes inconsistencies in blinding terminology, the use of double-blind design in randomized controlled trials, and the importance of the double-blind design in minimizing biases.

Blinding is used in various study designs but is most often associated with randomized controlled trials. Double-blind designs are used to minimize participant and researcher biases, which threaten the validity of a research study. Due to ambiguity in blinding terminology, researchers are encouraged to specifically report which individuals remain blind in a given study.

Blinding terminology varies across researchers, journals, and textbooks, leading to inconsistent definitions. Blinding, also known as masking, describes the process of withholding knowledge of intervention assignments from participants, investigators, or outcome assessors. Definitions of double blind and single blind disagree on which of these groups of individuals remain blind in each design. Participants, investigators, and assessors typically all remain blind in double-blind designs, but not all studies accord with this definition. To avoid confusion across definitions, researchers can replace basic terminology with specific descriptions of blinding procedures.

Double-blind designs are often associated with randomized controlled trials or studies where participants are randomly assigned to a treatment (intervention) or control (placebo). The placebo effect occurs when an individual’s behavior changes in response to a fake treatment (placebo) simply because that person expects a change. Double-blind designs control for the placebo effect because participants do not know whether they are receiving the treatment or placebo and therefore have equal expectations. Without blinding, participant biases may occur, where participants alter their behavior according to the results expected of their group. In educational research, for example, students who know they are placed in an academic intervention group may work harder to confirm expected academic improvements. Keeping students blind to group membership thus reduces potential participant biases.

Double-blind designs also minimize researcher biases, which occur when researchers (even unconsciously) influence results to confirm their expectations. When researchers allow their expectations to influence participant behavior, this creates self-fulfilling prophecies in participants who may confirm the study’s expected results. For example, a [Page 549]teacher in an intervention group who expects his or her students to improve academically may unconsciously provide his or her students with enhanced attention and enthusiasm.

Researcher biases also occur when researchers gather and interpret data in ways that might confirm their expectations. For example, an outcome assessor may look for and exaggerate academic improvements in an intervention classroom compared to a control classroom. Participant and researcher biases threaten internal validity, and double-blind designs can reduce this threat. Blinding procedures can therefore be effective in reducing bias but must be reported clearly in light of inconsistent definitions.

See also Internal Validity; Interviewer Bias; Placebo Effect; Random Assignment; Scientific Method; Selection Bias; Triple-Blind Studies; Validity