Experimenter Effect

via The Skeptic’s Dictionary – Skepdic.com

The experimenter effect is a term used to describe any of a number of subtle cues or signals from an experimenter that affect the performance or response of subjects in the experiment. The cues may be unconscious nonverbal cues, such as muscular tension or gestures. They may be vocal cues, such as tone of voice. Research has demonstrated that the expectations and biases of an experimenter can be communicated to experimental subjects in subtle, unintentional ways, and that these cues can significantly affect the outcome of the experiment (Rosenthal 1998).

Robert Rosenthal has found that even slight differences in instructions given to control and experimental groups can affect the outcome of an experiment. Different vocal intonations, subtle gestures, even slight changes in posture, might influence the subjects.


The experimenter effect may explain why many experiments can be conducted successfully only by one person or one group of persons, while others repeatedly fail in their attempts to replicate the results. Of course, there are other reasons why studies cannot be replicated. The original experimenter may have committed errors in design, controls, or calculations. Or he may have committed fraud.

Keep Reading: experimenter effect – The Skeptic’s Dictionary – Skepdic.com.

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