Is coefficient alpha robust to non-normal data?

Front Psychol. 2012 Feb 15;3:34. doi: 10.3389/fpsyg.2012.00034. eCollection 2012.

Abstract

Coefficient alpha has been a widely used measure by which internal consistency reliability is assessed. In addition to essential tau-equivalence and uncorrelated errors, normality has been noted as another important assumption for alpha. Earlier work on evaluating this assumption considered either exclusively non-normal error score distributions, or limited conditions. In view of this and the availability of advanced methods for generating univariate non-normal data, Monte Carlo simulations were conducted to show that non-normal distributions for true or error scores do create problems for using alpha to estimate the internal consistency reliability. The sample coefficient alpha is affected by leptokurtic true score distributions, or skewed and/or kurtotic error score distributions. Increased sample sizes, not test lengths, help improve the accuracy, bias, or precision of using it with non-normal data.

Keywords: Monte Carlo; coefficient alpha; error score distribution; kurtosis; non-normality; power method polynomials; skew; true score distribution.