In experimental psychology research, it is important to confirm that results of a study are actually due to independent or manipulated variables, rather than extraneous variables. Internal validity is the degree to which the results are associated with the independent variable, and not other, uncontrolled factors. When an experiment is said to be internally valid, a direct causal relationship between the independent and dependent variables is demonstrated unequivocally. This is in contrast to external validity, which is the generalizabilty of results across different experimental settings. This entry outlines several types of extraneous variables that may jeopardize the internal validity of research in educational psychology.

Threats to Internal Validity

If an experiment extends over a long period of time, uncontrolled changes may occur in the experimental groups—this confound ...

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