Input -output analysis is being widely used in developing countries for planning purposes. For a given level of final demand, input-output analysis allows us to project the required level of gross output to ensure consistency of plan. These projections are made on the assumption that the existing production structure is optimal and it implies that an increase in demand will be met through the expansion of domestic output even when it can be satisfied through an increase in imports. On the other hand, according to the semi-input-output method, we do not have to increase the output of international sectors in order to meet the increase in demand because the level and composition of these activities should be determined by comparative- cost considerations. These are the only national sectors in which output must increase in order to avoid shortage. The semi-input -output method has been such a useful and important contribution, yet, regrettably, its influence on the planning models had been rather limited.