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Validation: The process of testing and accumulating evidence that supports the valid use or interpretation of results from a measure or study.1

Validity: The degree to which a result (of a measurement or study) is likely to be true and free of bias (systematic errors).6 More broadly, the extent that accumulated evidence and theory is available to support the interpretation and use of results from a measure or study. See also internal validity, external validity. Types of validity are usually accompanied by a qualifying word or phrase; for example, in the context of measurement, expressions such as construct validity, face validity, content validity, and criterion validity are used.

Variable: A characteristic of interest in a study that has different values for different participants or objects.4 For example, variables could be sex (male or female), weight, or test scores.