Validity

Pre-requisites
Before reading this section, make sure you understand:
 * Correlation

Psychometric validity
Validity, in the context of psychometrics, refers to the extent to which a psychological test measures what it is purported to measure. Validity is commonly discussed in terms of there being three main types:
 * 1) Content validity
 * 2) Construct validity
 * 3) Criterion validity

In classical test theory, validity is limited by reliability - see also reliability and validity for more information.

Theoretical validity
Def. a "quality of a measurement indicating the degree to which the measure reflects the underlying construct, that is, whether it measures what it purports to measure" is called validity.

Reliability
Def.
 * 1) "the ability to measure the same thing consistently" or
 * 2) "(of a measurement indicating the degree to which the measure is consistent); that is, repeated measurements would give the same result"

is called reliability.