The concepts of validity and reliability are defined within the context of applied clinical research. Validity concerns what an instrument measures and how well it does so. Reliability concerns the faith that one can have in the data obtained from use of an instrument, that is, the degree to which any measuring tool controls for random error. The researcher selects a measure that demonstrates content, criterion, or construct validity, according to the intended use of the measurement tool. Similarly, the researcher selects a measure that demonstrates a degree of reliability that is needed for its application. It is possible to have reliability without validity, but it is logically impossible to demonstrate that an unreliable test is valid.