Unfortunately, the benefits of the regional input-output approach have their price. There are the numerous data problems outlined in chapter 3,especially the problems of disaggregating interindustry inputs and identifying their regional source; but for prediction, there is also the problems of the validity of the assumption of constant coefficients. The assumption that an industry will constantly use the same composition of inputs is undoubtedly dubious for many industrial sectors over anything but a very short time period. Changing technology, relative movements in price level,the development of economies of scale and a wide variety of social,psychological and political factors can all radically alter an industry’s input pattern. Trends in technology are particularly difficult to forecast although change dose appear to be occurring at an increasing rate. When,as in regional input-output analysis,the assumption of constant supply coefficients is also added, the whole approach looks rather problematic.