 The structural equations in a model specify which variables are to be held fixed at their actual values and how the values of other variables depend on one another. There is a structural equation for each variable. The form taken by a structural equation for a variable depends on which kind of variable it is. The structural equation for an exogenous variable (the values of which are determined by factors outside of the model) takes the form of Z = z, which simply states the actual value of the variable. The structural equation for an endogenous variable (the values of which are determined by factors within the model) states how the value of the variable is determined by the values of the other variables. It takes the form: