Figure 8–6 provides an excellent opportunity to see why the ROR method can result in selecting
the wrong alternative when only i * values are used to select between two alternatives. This is
sometimes called the ranking inconsistency problem of the ROR method. The inconsistency
occurs when the MARR is set less than the break even rate between two revenue alternatives.
Since the MARR is established based on conditions of the economy and market, MARR is established
external to any particular alternative evaluation. In Figure 8–6 the incremental break even
rate is 16.89%, and the MARR is 15%. The MARR is lower than break even; therefore, the incremental
ROR analysis results in correctly selecting filter 2. But if only the i * values were used,
filter 1 would be wrongly chosen, because its i * exceeds that of filter 2 (25.41% > 23.57%). This
error occurs because the rate of return method assumes reinvestment at the alternative’s ROR
value, while PW and AW analyses use the MARR as the reinvestment rate. The conclusion is
simple:
If the ROR method is used to evaluate two or more alternatives, use the incremental cash flows
and i* to make the decision between alternatives.