<P> For the investment - saving curve, the independent variable is the interest rate and the dependent variable is the level of income . (Note that economics graphs like this one typically place the independent variable--interest rate, in this example--on the vertical axis rather than the horizontal axis .) The IS curve is drawn as downward - sloping with the interest rate (i) on the vertical axis and GDP (gross domestic product: Y) on the horizontal axis . The initials IS stand for "Investment and Saving equilibrium" but since 1937 have been used to represent the locus of all equilibria where total spending (consumer spending + planned private investment + government purchases + net exports) equals an economy's total output (equivalent to real income, Y, or GDP). To keep the link with the historical meaning, the IS curve can be said to represent the equilibria where total private investment equals total saving, where the latter equals consumer saving plus government saving (the budget surplus) plus foreign saving (the trade surplus). In equilibrium, all spending is desired or planned; there is no unplanned inventory accumulation . The level of real GDP (Y) is determined along this line for each interest rate . </P> <P> Thus the IS curve is a locus of points of equilibrium in the "real" (non-financial) economy . Each point on the curve represents the equilibrium between the Savings and Investment (S = I). </P> <P> Given expectations about returns on fixed investment, every level of the real interest rate (i) will generate a certain level of planned fixed investment and other interest - sensitive spending: lower interest rates encourage higher fixed investment and the like . Income is at the equilibrium level for a given interest rate when the saving that consumers and other economic participants choose to do out of this income equals investment (or, equivalently, when "leakages" from the circular flow equal "injections"). The multiplier effect of an increase in fixed investment resulting from a lower interest rate raises real GDP . This explains the downward slope of the IS curve . In summary, this line represents the causation from falling interest rates to rising planned fixed investment (etc .) to rising national income and output . </P> <P> The IS curve is defined by the equation </P>

An explanation for the slope of the is curve is that as the interest rate increases
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