ABSTRACT
With regard to the experimental research of interest to us here, internal validity can therefore be defined as the degree to which the experiment demonstrates the actual cause–effect relationship between the independent variable (manipulated by the experimenter during the experiment) and the results, or the behaviors displayed by the participants. In other words, internal validity can be understood as the degree to which the results are attributable to the independent variable and not some other rival explanation. Another important feature of an experiment is external validity. The interaction of historical events and membership in experimental groups can be a source of disruption to internal validity. Relatively rarely is attention turned to the attrition factor within the context of the internal validity. Experimental psychologists are well aware of the complexity of what they are investigating, and they very carefully try to control the influence of the various factors on the dependent variable.
