Scale quality: alpha is an inadequate estimate and factor-analytic evidence is needed firstly

Abstract

Background: Cronbach’s alpha is a commonly reported estimate to assess scale quality in health psychology and related disciplines. However, despite being frequently reported as such, alpha is unrelated to a scale’s internal structure. Methods: We have screened all articles published in Psychology & Health last year. A total of 288 scales were reported in 88 articles. For 233 of these scales (80.9%), an estimate of scale quality was reported, which was alpha for 210 scales (90.1%). Of the 288 scales we surveyed, authors assessed dimensionality for 10 scales (2.4%). Findings: We argue that alpha is an inadequate estimate for both validity and reliability – two key elements of scale quality. Omega is a readily available alternative that can be used for both interval and ordinal data. More importantly, we argue that factor-analytic evidence should be presented before assessing the internal structure of a scale. Discussion: Pointers for readers and reviewers of manuscripts on making judgements about scale quality are provided and illustrated by examples from the field of health psychology.