The extent to which a measurement, test or study measures what it purports to measure. There are various types of validity: construct validity: The degree to which an instrument measures the characteristic being investigated; the extent to which the conceptual definition matches the operational definition. content validity: Verification that the method of measurement actually measures what it is expected to measure, covering all areas under investigation reasonably and thoroughly. criterion validity: Verification that the instrument correlates with external criteria of the phenomenon under study, either concurrently or predictively. external validity: The extent to which study results can be generalized beyond the sample used in the study. face validity: A type of content validity, determining the suitability of a given instrument as a source of data on the subject under investigation, using commonsense criteria. internal validity: the extent to which the effects detected in a study are truly caused by the treatment or exposure in the study sample, rather than being due to other biasing effects of extraneous variables.
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