In survey analysis, e.g. in psychometrics, reliability is a measure of reproducibility of the survey instrument or test. In other words, reliability is a measure of precision – i.e. it describes the random error of the survey instrument.
There are several types of reliability: test-retest reliability, alternate-form reliability, internal consistency reliability, Internal consistency reliability. intraobserver reliability, interobserver reliability.
Note: the term reliability has a different meaning in natural sciences and engineering (see reliability).