Double-blind experiments

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 rigor.

In a double-blind experiment, neither the individuals nor the researchers know who belongs to the control group and the experimental group. Only after all the data have been recorded (and in some cases, analyzed) do the researchers learn which individuals are which. Performing an experiment in double-blind fashion is a way to lessen the influence of the prejudices and unintentional physical cues on the results (the placebo effect, observer bias, and experimenter's bias). Random assignment of the subject to the experimental or control group is a critical part of double-blind research design. The key that identifies the subjects and which group they belonged to is kept by a third party and not given to the researchers until the study is over.

Double-blind methods can be applied to any experimental situation where there is the possibility that the results will be affected by conscious or unconscious bias on the part of the experimenter.
A blind or blinded experiment is a scientific experiment where some of the persons involved are prevented from knowing certain information that might lead to conscious or unconscious bias on their part, invalidating the results.

For example, when asking consumers to compare the tastes of different brands of a product, the identities of the latter should be concealed — otherwise consumers will generally tend to prefer the brand they are familiar with. Similarly, when evaluating the effectiveness of a medical drug, both the patients and the doctors who administer the drug may be kept in the dark about the dosage being applied in each case — to forestall any chance of a placebo effect, observer bias, or conscious deception.

Blinding can be imposed on researchers, technicians, subjects, funders, or any combination of them. The opposite of a blind trial is an open trial. Blind experiments are an important tool of the scientific method, in many fields of research — from medicine, forensics, psychology and the social sciences, to basic sciences such as physics and biology and to market research. In some disciplines, such as drug testing, blind experiments are considered essential.

The terms blind (adjective) or to blind (transitive verb) when used in this sense are figurative extensions of the literal idea of blindfolding someone. The terms masked or to mask may be used for the same concept. (This is commonly the case in ophthalmology, where the word 'blind' is often used in the literal sense.)

Biases can occur in any one of seven stages of research:

1.in reading-up on the field,
2.in specifying and selecting the study sample,
3.in executing the experimental manoeuvre (or exposure),
4.in measuring exposures and outcomes,
5.in analyzing the data,
6.in interpreting the analysis, and
7.in publishing the results.
The inability of a human being to remain completely objective is the ultimate source of this bias. It occurs more often in sociological and medical sciences, for which reason double blind techniques are often employed to combat the bias. But experimenter's bias can also be found in some physical sciences, where the experimenter rounds off measurements.