The CALIS procedure provides six methods of estimation specified by the METHOD= option:
DWLS 
diagonally weighted least squares 

FIML 
fullinformation maximum likelihood 

GLS 
normal theory generalized least squares 

ML 
maximum likelihood for multivariate normal distributions 

ULS 
unweighted least squares 

WLS 
weighted least squares for arbitrary distributions 
Each estimation method is based on finding parameter estimates that minimize a discrepancy (badnessoffit) function, which measures the difference between the observed sample covariance matrix and the fitted (predicted) covariance matrix, given the model and the parameter estimates. The difference between the observed sample mean vector and the fitted (predicted) mean vector is also taken into account when the mean structures are modeled. See the section Estimation Criteria in Chapter 27: The CALIS Procedure, for formulas, or see Loehlin (1987, pp. 54–62) and Bollen (1989, pp. 104–123) for further discussion. For the ML method, robust estimation with the ROBUST option is also available.
The default estimation is METHOD=ML, which is the most popular method for applications. The option METHOD=GLS usually produces very similar results to those produced by METHOD=ML. If your data contain random missing values and it is important to use the information from those incomplete observations, you might want to use the FIML method, which provides a sound treatment of missing values in data. METHOD=ML and METHOD=FIML are essentially the same method when you do not have missing values (see Example 27.16 of Chapter 27: The CALIS Procedure,). Asymptotically, ML and GLS are the same. Both methods assume a multivariate normal distribution in the population. The WLS method with the default weight matrix is equivalent to the asymptotically distribution free (ADF) method, which yields asymptotically normal estimates regardless of the distribution in the population. When the multivariate normal assumption is in doubt, especially if the variables have high kurtosis, you should seriously consider the WLS method. When a correlation matrix is analyzed, only WLS can produce correct standard error estimates. However, in order to use the WLS method with the expected statistical properties, the sample size must be large. Several thousand might be a minimum requirement.
The ULS and DWLS methods yield reasonable estimates under less restrictive assumptions. You can apply these methods to normal or nonnormal situations or to covariance or correlation matrices. The drawback is that the statistical qualities of the estimates seem to be unknown. For this reason, PROC CALIS does not provide standard errors or test statistics with these two methods.
You cannot use METHOD=ML or METHOD=GLS if the observed covariance matrix is singular. You can either remove variables involved in the linear dependencies or use less restrictive estimation methods such as ULS. Specifying METHOD=ML assumes that the predicted covariance matrix is nonsingular. If ML fails because of a singular predicted covariance matrix, you need to examine whether the model specification leads to the singularity. If so, modify the model specification to eliminate the problem. If not, you probably need to use other estimation methods.
You should remove outliers and try to transform variables that are skewed or heavytailed. This applies to all estimation methods, since all the estimation methods depend on the sample covariance matrix, and the sample covariance matrix is a poor estimator for distributions with high kurtosis (Bollen 1989, pp. 415–418; Huber 1981; Hampel et al. 1986). PROC CALIS displays estimates of univariate and multivariate kurtosis (Bollen, 1989, pp. 418–425) if you specify the KURTOSIS option in the PROC CALIS statement.
See the section Estimation Methods for the general use of these methods. See the section Estimation Criteria of Chapter 27: The CALIS Procedure, for details about these estimation criteria.