The data structures that are produced when one creates an OpenMx SEM model are a departure from the structures produced by other SEM software. We will next describe these structures and how they fit together. While software has improved, SEM modelers continue to think about their model structure in ways that have changed very little since the 1960s. One may use OpenMx without changing one's conception of model building, continuing to use path specifications or matrix specifications in a serially ordered script. However, the fact that R is interactive, has powerful vector and matrix operations, and incorporates the flow control of a full programming language all act to allow one to rethink the way models are specified. The OpenMx data structures are designed to flexibly accommodate the power of R. The authors hope that these factors will be sufficient to trigger a paradigm shift in the way SEM is conceived and taught.
