# 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:
* Content validity
* Construct validity
* Criterion validity
In classical test theory, validity is limited by reliability - see also reliability and validity for more information.

## Content validity

Content validity is the degree to which the content of the test matches the content domain of the target construct.
For example, a test of ability to add two-digit numbers should cover the full range of combinations of digits (10 to 99). A test with only one-digit numbers, or only even numbers, would not have good coverage of the content domain.
Content validity evidence typically involves subject matter experts evaluating test items against the test specifications. Such expert-based testing of content validity is distinguished from the simpler notion of face validity.

## Criterion validity

Criterion validity involves the correlation between the test and a criterion variable (or variables) which are taken as representative of the construct.
There are two types of criterion validity:
* Concurrent validity: Test data and criterion data are collected at the same time, e.g.,
** Self-report and observer measures collected at the same time
* Predictive validity: Test data is collected first in order to predict criterion data collected at a later point in time, e.g.,
** Employee selection tests are often validated against measures of job performance.
** Measures of risk of recidivism among those convicted of a crime can be validated against measures of recidivism.

## Construct validity

Construct validity refers to the totality of evidence about whether a particular operationalisation of a construct adequately represents what is intended by the theoretical account of the construct being measured.
Construct validity includes empirical and theoretical support for the interpretation of the construct. Lines of evidence include statistical analyses of the internal structure of the test (including factor analysis and internal consistency) and correspondence between the test measure and the target construct as judged by experts (content validity).
Construct validity is not distinct from the support for the substantive theory of the construct that the test is designed to measure. There are two main approaches to construct validity:
* Convergent validity: Degree to which a measure is correlated with other measures that it is theoretically predicted to correlate with.
* Discriminant validity: Degree to which the operationalisation does not correlate with other operationalisations that it theoretically should not correlate with.

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