Experimenter effects and internal validity

An experimenter effect, which results in experimenter bias, can threaten internal validity across all types of experimental and quasi-experimental research design. Such an experimenter effect is typically unintentional, but arises because of (a) the personal characteristics of the researcher, which influences the choices made during a study; and (b) non-verbal cues that the researcher gives out that may influence the behaviour and responses of participants. Some of the more generic personal characteristics that may lead to bias include the experimenter's age, class, gender, race, and so forth. It can sometimes help to think about experimenter effects as relating to either implementation/research methods or directional hypotheses/personal biases. Each is discussed in turn:

Subject effects

Subject effects (or participant reactivity) occur when the way that participants behave in an experiment is different from the way that they would normally behave. These changes in behaviour reflect participants? knowledge that they are being studied, which may lead to them acting aggressively/defensively, cooperatively/uncooperatively, or in some other way that affects their score on the dependent variable. Participants may behave differently in order to mirror the behaviour that they think the researcher wants to see, or they may do it for their own reasons. Nonetheless, this behavioural modification can threaten the internal validity of the study because the way that participants reacted may explain the changes in the dependent variable rather than the treatment (i.e., the independent variable).

Subject effects are likely to be greater in staged experiments where it is particularly obvious that subjects are part of an experiment (e.g., a laboratory settings), compared with less staged environments where participants know that they are part of an experiment, but are not so "under the microscope".


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