Macro: Keynesian investment function


I
t = AI –f(i)

where:

AI          intercept of the investment function, or what investment expenditures would be if interest rates were zero

f            slope of the investment function, or the change in investment with respect to a change in the interest rate (known as the marginal efficiency of investment)

i             rate of interest

It           gross investment expenditures

Notice something missing here? Income.