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Chapter 45: Internal and External Validity
Internal validity and external validity are two sets of criteria that be used in evaluating the worthiness of an experimental design. Internal validity is the quality of an experimental design such that the results obtained can be attributed to the manipulation of the independent variable, whereas external validity is the quality of an experimental design such that the results can be generalized from the original sample and by extension, to the population from which the sample originated. Along with their seminal contribution to understanding and designing experiments, Campbell and Stanley (1963) identified threats to both internal and external validity.
The following is a brief description of threats to the internal validity of an experiment.