Structural equation models are used by biologists, educational and medical researchers, psychologists, social scientists and others who traditionally deal with nonexperimental and quasi-experimental data. Perhaps the most important and influential statistical revolution to have recently occurred in the scientific arena, the development of structural equation models has provided researchers with a comprehensive method for the quantification and testing of theories. Accepted today as a major component of applied multivariate analysis, structural equation modelling includes latent variables, measurement errors in both dependent and independent variables, multiple indicators, reciprocal causation, simultaneity and interdependence. As implemented in most commerical computer packages - Amos, EQS, LISREL, LISCOMP, Mx, SAS PROC-CALIS, STATISTICA-SEPATH - the method includes as special cases such procedures as confirmatory factor analysis, multiple regression, path analysis, models for time-dependent data, recursive and non-recursive models for cross-sectional and longitudinal data, and covariance structure analysis.