Although active, initial, and data equations all result in the same form of error, the true source and solution of the errors might differ.
•Active simultaneous equations represent some genuinely incorrect representation, often a conceptual error. Somewhere there are additional dynamics separating things and you must explicitly include these. If the simultaneity is intentional, you must either solve the equation outside of Vensim or make use of an iterative solution technique.
•Simultaneous initial condition equations usually result from a need to initialize the model in equilibrium or a representative condition. You can usually solve such equations by breaking the chain and setting one of the variables to a known or reasonable value using, for example, an ACTIVE_INITIAL function (in the Equation Editor you specify the variable to be an Auxiliary with Initial).
•Simultaneous data equations are generally the result of typographical errors, although they can bear some resemblance to simultaneous active equations.
To get an understanding of simultaneous equations, it often helpful to just walk through the list of equations by clicking on the different variables mentioned in the list. Another trick that can sometimes be helpful is to make a new view and add in the variables involved in the loop leaving other variables as shadow variables.
When you do make changes to resolve a set of simultaneous equations, you might uncover another set. This happens because when a simultaneous equation is first identified, the variables involved will not be searched further until the problem has been corrected. Once corrected, another search might uncover another set of simultaneous equations.
