Behavioural and Technical Change in Economic Models

Techniques of sector planning is a large subject on which it would be difficult to get beyond generalities in a short paper. Our work on this subject is at the model-building stage and we have recently described its general lines in [1].2 Here we shall select one aspect to write about: the problem of freeing economic models from their dependence on parameters appropriate to the past. We all know that tastes and techniques change, that new commodities and processes are introduced, but too often we continue to view the future through the spectacles of the past as if none of these things ever happened. These problems become all the more insistent in disaggregated, sectoral models. What, then, are we to do about them ?