In the theory of investment decisions, the most appropriate rate of return is the one which makes the :

The internal rate of return is the discount rate that makes the net present value of a project's cash flows equal to zero; Keynes's marginal efficiency of capital is defined in the same way. A project is worth undertaking if this rate exceeds the cost of capital. Maximising net present value is a rule for choosing among projects at a given discount rate, not a definition of the rate of return.