A simulation study investigated the effects of skewness and kurtosis on level-specific maximum likelihood (ML) test statistics based on normal theory in multilevel structural equation models. The levels of skewness and kurtosis at each level were manipulated in multilevel data, and the effects of skewness and kurtosis on level-specific ML test statistics were examined. When the assumption of multivariate normality was violated, the level-specific ML test statistics were inflated, resulting in Type I error rates that were higher than the nominal level for the correctly specified model. Q-Q plots of the test statistics against a theoretical chi-square distribution showed that skewness led to a thicker upper tail and kurtosis led to a longer upper tail of the observed distribution of the level-specific ML test statistic for the correctly specified model.
Keywords Maximum likelihood test statistic . Violation of multivariate normality . Multilevel structural equation modelingStructural equation modeling (SEM) has become an important and widely used analysis approach in social and behavioral science over the past 3 decades (Bollen, 2002;MacCallum & Austin, 2000). SEM (Bentler, 1980;Jöreskog, 1978) is a general multivariate technique that accounts for the means, variances, and covariances of a set of variables in terms of a smaller number of parameters associated with a hypothesized model.One of the assumptions made in SEM is that the observations are independent. When data have multilevel structure such that individuals are nested within clusters, the independence assumption typically is violated, because individuals in the same cluster are likely to be more homogeneous than those from different clusters. Multilevel SEM has been developed to apply SEM to multilevel data, dealing with non independent observations by explicitly modeling the clustered nature of multilevel data (Goldstein & McDonald, 1988;Lee, 1990;Longford & Muthén, 1992;Muthén, 1990Muthén, , 1994Muthén & Satorra, 1995).The typical application of SEM and multilevel SEM has interest in both (1) assessing the goodness of fit of the model to the data and (2) estimating and testing individual parameters in the hypothesized model. The most widely used method for estimation and testing is maximum likelihood (ML) estimation based on normal theory. ML estimation finds the set of parameter estimates that maximizes the likelihood that the data will be observed, given that the data have a multivariate normal distribution in the population. ML estimation also provides the likelihood ratio test statistic, which provides a test of whether the hypothesized model fits the data. The multivariate normality assumption is important for the ML estimator to achieve its asymptotic properties and for statistical inference to be valid.The present study investigated how skewness and kurtosis affect the performance of the normal-theory ML test statistic for overall model fit in multilevel structural equation models, under violations of the multivariate normality assumpti...