The Profitability Index (PI) is defined as the ratio of the net present value to initial outlay.
Profitability Index = 
(3.8)[15]
The opportunity cost of capital is 10 percent and company has the following opportunities:
| Cash Flows ($) | ||||||
| Project | C0(investment) | C1 | C2 | NPV at 10% | PI | |
| A | -10 | +30 | +5 | 2.1 | ||
| B | -5 | +5 | +20 | 3.2 | ||
| C | -5 | +5 | +15 | 2.4 | ||
| D | -40 | +60 | 0.4 | |||
Table 3.11
The projects A, , C are attractive, but the firm is limited to spending $ 10 million. In that case, it can invest either in project A or in projects and C, but it cannot invest in all three. In this case, manager cannot choose between projects solely on the basis of net present value. When founds are limited, manager must pick the project that offer the highest net present value per dollar of initial outlay. This ratio is PI. Manager is calculated PI in table and then manager can see that project has highest PI and C has next highest. Therefore, manager should accept these two projects, manager have budget limit $10 million.
If the project has positive PI, it must also have a positive NPV. Therefore, firm use the PI to select projects when capital is not limited. However, the PI can be misleading when used to choose between mutually exclusive projects. Unfortunately, there is some limitations simple ranking method. One of the most serious is that it breaks down whenever more than one resource is rationed. It may also break down if it causes some money to be left over. It might be better to spend all the available founds even if this involves accepting a project with a slightly lower NPV. Suppose in our example that the firm can spend only $10 million for invest in each of years 0 and 1 and that the menu of possible projects is expanded to include an investment next year in project D. When manager choose A, it provides to take a $30 million positive cash flow in period 1 and it invested money in project D. A and D have lower PI than and C, but they have a higher total NPV. The reason that the PI fails in our example is that resources are constrained in each of two periods. In fact, this ranking method is inadequate whenever there is any other constraint on the choice of projects. This means that batter do not cope with case in which two projects are mutually exclusive or in which on project is depend on another. One way to solve such problem is work through all possible combinations of project. But the liner programming techniques are specially created to search through such possible combinations.
In developing the NPV rule, manager mast means that the firm can maximize shareholder wealth by accepting every project that is worth more than it costs. But, if capital is strictly limited, then it may be possible to take every project with positive NPV. If capital is rationed in only one period, then the firm should follow the rule: calculate each projects NPV per dollar of investment. Then pick the project with the highest PI until manager run out capital. The procedure fails when capital is rationed in more then one period or when there are other constraints on project choice. The only general solution is linear programming (LP).
