Estimation of reliability: a better alternative for Cronbach's alpha

We study the estimation of the reliability of measurement scales. The subject has been partially obscured because of poor estimators, such as Cronbach’s alpha, which is the most widely applied estimator of reliability. We show that a recently proposed estimator that we are calling Tarkkonen’s rho, provides a general method for the task. Examining the assumptions behind these estimators we prove that Cronbach’s alpha is a special case of Tarkkonen’s rho under certain one-dimensional measurement models. In practice, the conditions required for Cronbach’s alpha to be a valid estimator of reliability are difficult to encounter, whereasTarkkonen’s rho is well suited for more general measurement settings. Our conclusion is that Tarkkonen’s rho is a better alternative for Cronbach’s alpha.