9 Tobin's q, or the ratio of the market value of a firm's assets (as measured by the market value of its outstanding stock and debt) to the replacement cost of the firm's assets, is used as an ordinary investment function model. As a result of the optimization based on a firm's production function, Tobin's q can be formulated as the ratio of the marginal capital productivity to capital cost (see Hanasaki (2002)). The investment function model for this analysis is based on Hanasaki (2002).