Coefficient alpha: a useful indicator of reliability?

A series of Monte Carlo simulations were carried out to examine the performance of Cronbach’s alpha as an index of reliability. Data were generated to be consistent with a single factor measured with six items. The magnitude of the factor loadings, systematic error and sample size were manipulated and alpha calculated from random samples. The results showed that alpha is influenced by factors other than the reliability of the items that comprise a scale. In particular the amount of systematic error, or deviation from unidimensionality, increased the estimate of alpha. The results are discussed in terms of traditional interpretation of alpha.