MACRO-ECONOMIC MODEL EVALUATION 31 through judicious modifications and use of constant adjustments. However, it is important for the wider acceptance of the worth of such a model that it be subjected to outside scrutiny in an unmodified form.
BACKGROUNDMost econometric models are developed with two objectives in mind: for prediction under a range of shocks; and for policy prescription.Other objectives, such as use as a teaching tool or to discriminate between alternative theories, are usually of lesser interest in practice.While it may be desirable to place the evaluation process into an explicit decision theoretical framework. this has proved quite difficult, even in the single equation case.y Thus, a n essential part of the evaluation will centre around standard (frequentist) statistical properties of the model. More precisely, what statistical criteria can be used to determine whether a model is useful or even whether one model is better than another for a given task? A natural way to develop such tests would seem to rely on properties of dynamic simulations of the complete system (i.e. along the lines of various tests for individual equations which are based on either within sample properties or the predictive ability of the model). Unfortunately, as has been argued effectively by Pagan (1986) and Chong and Hendry (1986), dynamic simulations may be of limited use and indeed, at times, cloud the issues."' In particular, consider the linear model: which, given A ( L ) is invertible, has a final form:The residuals from the dynamic simulations are then: fit = ( A ( L ) ) -l u t .
( 3 )Under general conditions the fit are linear combinations of the u t . Thus, the dynamic simulation residuals in principle convey n o information that is not present in the errors of the structural equations. In addition, it is clear from equation (3) that the fitwill be serially correlated making the development of formal tests difficult.Accordingly, tests on dynamic residuals from the system will be at best difficult. Consequently, formal statistical evaluation in NIF88 relies heavily on the testing of 'See Efron (1986), and the subsequent comments, for a discussion of why Irequentist techniques continue to dominate practical work."These authors emphasise that dynamic simulations will have their uses. including facilitating an understanding of the long-run properties of the models. However, they argue that they are not useful for more formal statistical validation.