The ability of a research design to represent the true causal relationship between an intervention and an effect in the particular circumstances of the research.
Note: If the internal organization of a study leads to distortion in the estimation measurement, that measurement lacks internal validity. This is what happens when, for example, a person is weighed using a defective scale. As with a well-calibrated scale, an etiological study must be free of measurement-distorting internal defects. These defects may be encountered at three stages in the study: the subject selection, information-gathering and analysis stages.