Incremental IRR (Internal Rate of Return)

The Incremental IRR

Despite their apparent wealth maximization defects, IRR project rankings that conflict with NPV
can be brought into line by a  supplementary  IRR procedure whereby management: 

Determine the incremental yield (IRR) from an  incremental investment,
which measures marginal profitability by subtracting one project’s cash
inflows and outflows from those of another to create a  sub-project 
(sometimes termed a  ghost or  shadow project).

To prove the point, let us incremental the data from Section 3.1.Two projects that not only
differ with respect to their cash flow patterns ( size  and  timing ) but also their investment cost.

Project  Year 0  Year 1  Year 2  Year 3  Year 4  Year 5  IRR(%) NPV
                                                                                   15%        (10%)  
1 less 2  (35)      (30)     -           20     40   50   11.1

You will recall that IRR maximization favored a higher  percentage return on the smaller more
liquid investment (Project1), whereas NPV maximization focused on higher money profits
overall (Project 2). Now see how the incremental IRR (15%) on the incremental investment
(Project 1 minus Project 2 = £35k) exceeds the discount rate (10%) so Project 1 is accepted.
Moreover, this corresponds to Equation (1) on single project acceptance. The incremental NPV is
positive (£11.1k) because its discount rate r < incremental IRR.

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