The reliability of the growth rate parameter estimator first presented by J. B. Willet for a linear growth model is derived and critically examined. The effects on the reliability of the growth rate of the intervals between the longitudinal measurements and the covariances between the longitudinal measurements are distinguished. It is demonstrated that, mathematically, the latter and not the former functionally determines the reliability of the growth rate, but it is also recognized that, practically, the time intervals may affect the covariances.
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