What is reliability versus validity?

Reliability is the consistency of a measure; validity is whether it measures what it claims to. A test can be reliable without being valid.

Psychometrics & Statistics & Methodology

Psychometrics is the science of psychological measurement — the theory and methods for constructing, evaluating, and interpreting tests and scales.

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Scope

It covers test theory (classical and item-response), reliability and validity, factor analysis, and the statistical methodology of psychological research.

Sub-topics

Core questions

How can psychological attributes be measured?
How reliable and valid is a measure?
How should tests be constructed and scored?
How is measurement error modelled?

Key concepts

Reliability
Validity
Factor analysis
Item response theory
Measurement error
The g factor

Key theories

Factor analysis and intelligence: Spearman introduced factor analysis and the general-intelligence factor g.
Reliability: Cronbach's alpha became the standard index of internal-consistency reliability.
Construct validity: Cronbach and Meehl formalized construct validity, central to measurement theory.

History

From Galton and Spearman's origins, psychometrics developed classical test theory (reliability, validity), factor analysis, and later item-response theory, providing the measurement foundations of psychology.

Debates

How should validity be conceptualized?: Construct validity reframed validity as a unified, theory-laden judgment rather than a set of separate coefficients.

Key figures

Charles Spearman
Lee Cronbach
Paul Meehl

Seminal works

spearman-1904
cronbach-1951
cronbach-meehl-1955

Frequently asked questions

What is reliability versus validity?: Reliability is the consistency of a measure; validity is whether it measures what it claims to. A test can be reliable without being valid.