The alpha and the omega of scale reliability and validity: why and how to abandon Cronbach’s alpha and the route towards more comprehensive assessment of scale quality

Health Psychologists using questionnaires rely heavily on Cronbach’s alpha as indicator of scale reliability and internal consistency. Cronbach’s alpha is often viewed as some kind of quality label: high values certify scale quality, low values prompt removal of one or several items. Unfortunately, this approach suffers two fundamental problems. First, Cronbach’s alpha is both unrelated to a scale's internal consistency and a fatally flawed estimate of its reliability. Second, the approach itself assumes that scale items are repeated measurements, an assumption that is often violated and rarely desirable. The problems with Cronbach’s alpha are easily solved by computing readily available alternatives, such as the Greatest Lower Bound or Omega. Solving the second problem, however, is less straightforward. This requires forgoing the appealing comfort of a quantitative, seemingly objective indicator of scale quality altogether, instead acknowledging the dynamics of reliability and validity and the distinction between scales and indices. In this contribution, I will explore these issues, and provide recommendations for scale inspection that takes these dynamics and this distinction into account.