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Glossary
The consistency of an assessment's results when the same person takes it twice, separated by time.
Test-retest reliability is the consistency of an assessment's results when the same person takes it twice, separated by a period of time. High test-retest reliability indicates the score reflects a stable trait or characteristic rather than a transient state.
The interval matters: a few days is typical for trait measures; for state measures (like current mood), test-retest reliability is intentionally lower because the underlying state is expected to change. Reliability is usually reported as a correlation coefficient (Pearson's r or intraclass correlation).