The key assumption of the Solow–Swan growth model is that capital is subject to diminishing returns in a closed economy. The Solow residual is the portion of an economy's output growth that cannot be attributed to the accumulation of capital and labor, the factors of production.An increase in saving and investment raises the capital stock and thus raises the full-employment national income and product. Output and capital per worker grow at the same constant, positive rate in BGP of model. In long run model reaches BGP. 2. In the forecasting of economic growth, understanding the likely development of capital and labor stocks is important.