In many cases we are interested in only one (or a few) of the equations of the model and attempts to measure its parameters statistically without a complete knowledge of the entire model. Under these circumstances as the structure of the model is not known, identification of the relationship cannot be established by applying the order and rank conditions. In such cases ideritification is established imposing certain restrictions on the values of some parameters in some equations.

The restriction on the values ol" the parameters can be in any of the following forms:

1) Zero restrictions.

2) Equality restrictions, or other linear relationship between some of the parameters.

3) Exi. -neous estimates of some parameters.