## Macro: Keynesian investment function

I_{t}**= A**_{I}**–f(i)**

**where:**

**A**_{I}** 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**

**I**_{t}** gross investment expenditures**

Notice something missing here? *Income*.