Internal validity refers to the certainty that experimental interventions did indeed cause the changes observed in the study group; also the control over confounding factors which tend to invalidate the results of an experiment.
Other definition:
Internal validity refers to the extent to which an intended manipulation of a variable, rather than extraneous influences, accounts for observed results, changes, or Group differences.
Other /More definition:
internal validity refers to extent to which the results of a study can be attributed to the independent variable after confounding alternative explanations have been ruled out.

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