Cronbach’s alpha

assessing scale reliability

Cronbach’s alpha Cronbach's alpha is one of the most

common measures of reliability. It measures the internal consistency

of the items in a scale. It indicates the extent to which the

items in a questionnaire are related to each other

It indicates whether a scale is unidimensional or multidimensional

Interpreting scale reliability The higher the alpha, the more reliable

the scale. A score of .70 or greater is generally

considered to be acceptable◦ .90 or > = high reliability◦ .80-.89 = good reliability◦ .70-79 = acceptable reliability◦ .65-.69 = marginal reliability

lower thresholds are sometimes used.

Example of an alpha score The Happiness Subscale of the Short Version of the

Adolescent General Well-Being (AGWB) Scale (Columbo, 1984/1986)

The “happiness subscale” is a 9-item, 5-point Likert scale that assesses adolescents’ personal experience of happiness.

Scores range from 9 to 45, with higher scores indicating greater happiness. The population of intended use is adolescents aged 14 to 18 years. Empirical studies have suggested the internal consistency of the scale with

alphas ranging from .87 to .90 (Mahon & Yarcheski, 2002).