Blind and Open Experiments and Observer Bias

In true science there is a difference between a blind experiment and open experiment.  The blind method is a part of the scientific method.

The blind experimental model is used to prevent research outcomes from being influenced by observer bias.  Observer bias is also called observer-expectancy effect (also called the experimenter-expectancy effect, observer effect, contextual bias or experimenter effect).  Observer bias is a form of reactivity, in which a researcher’s bias causes them to unconsciously influence the participants of an experiment.

It is the most significant threat to a study’s internal validity.

It is therefore typically controlled using a double-blind experimental design.

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.  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.

  • 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.
  • Double-blind describes an especially stringent way of conducting an experiment in an attempt to eliminate subjective bias on the part of both experimental subjects and the experimenters.

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).  The random assignment of the subject to the experimental or control group is a critical part of double-blind research design.

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