The effects of correlated errors on generalizability and dependability coefficients

This study investigated the effects of correlated errors on the person x occasion design in which the confounding effect of equal time intervals results in correlated error terms in the linear model. Two specific error correlation structures were examined: the first-order stationary autoregressive (SARI), and the first-order nonstationary autoregressive (NARI) with increasing variance parameters. The effects of correlated errors on the existing generalizability and dependability coefficients were assessed by simulating data with known variances (six different combinations of person, occasion, and error variances), occasion sizes, person sizes, correlation parameters, and increasing variance parameters. Estimates derived from the simulated data were compared to their true values. The traditional estimates were acceptable when the error terms were not correlated and the error variances were equal. The coefficients were underestimated when the errors were uncorrelated with increasing error variances. However, when the errors were correlated with equal vanances the traditional formulas overestimated both coefficients. When the errors were correlated with increasing variances, the traditional formulas both overestimated and underestimated the coefficients. Finally, increasing the number of occasions sampled resulted in more improved generalizability coefficient estimates than dependability coefficient estimates. Index terms: changing error variances, computer simulation, correlated errors, dependability coefficients, generalizability coefficients