Validity of a study explains how much of the change in the dependent variable was due to the independent variable. If validity is high, most of the change was due to the independent variable. In testing, a test is considered to have high validity if it measures what it’s supposed to measure. In statistics, validity can be of two types, internal validity and external validity. Internal Validity refers to whether the effects on the dependent variable are due to the manipulation in the independent variable and not some other factors and if the relationship is a causal one. External Validity relates to the possibility that the relationship between two variables, measured in the experiment, can be generalized to other settings, other people, and over time.
Even though internal validity is necessary to establish external validity, having high internal validity does not guarantee external validity.
Validity cannot exceed the square-root of Reliability.