Definition

Reliability

The consistency of a measure — across items (internal consistency), time (test–retest), and raters (inter-rater).

A reliable measure gives similar results under similar conditions. Internal consistency (Cronbach’s alpha ≥ .70 by convention) checks that scale items hang together; test–retest checks stability; inter-rater agreement (Cohen’s kappa) checks that judges see the same thing.

Reliability caps validity: a noisy measure cannot measure anything well. But high reliability does not guarantee validity — a bathroom scale that is consistently 5 kg off is reliable and wrong.