When the calculated IRR is higher than the true reinvestment rate for interim cash flows, the measure will overestimate — sometimes very significantly — the annual equivalent return from the project. The formula assumes that the company has additional projects, with equally attractive prospects, in which to invest the interim cash flows. In this case, the calculation implicitly takes credit for these additional projects. Calculations of net present value (NPV), by contrast, generally assume only that a company can earn its cost of capital on interim cash flows, leaving any future incremental project value with those future projects.
IRR’s assumptions about reinvestment can lead to major capital budget distortions. Consider a hypothetical assessment of two different, mutually exclusive projects, A and B, with identical cash flows, risk levels, and durations — as well as identical IRR values of 41 percent. Using IRR as the decision yardstick, an executive would feel confidence in being indifferent toward choosing between the two projects. However, it would be a mistake to select either project without examining the relevant reinvestment rate for interim cash flows. Suppose that Project B’s interim cash flows could be redeployed only at a typical 8 percent cost of capital, while Project A’s cash flows could be invested in an attractive follow-on project expected to generate a 41 percent annual return. In that case, Project A is unambiguously preferable.
Even if the interim cash flows really could be reinvested at the IRR, very few practitioners would argue that the value of future investments should be commingled with the value of the project being evaluated. Most practitioners would agree that a company’s cost of capital — by definition, the return available elsewhere to its shareholders on a similarly risky investment — is a clearer and more logical rate to assume for reinvestments of interim project cash flows.
When the cost of capital is used, a project’s true annual equivalent yield can fall significantly — again, especially so with projects that posted high initial IRRs. Of course, when executives review projects with IRRs that are close to a company’s cost of capital, the IRR is less distorted by the reinvestment-rate assumption. But when they evaluate projects that claim IRRs of 10 percent or more above their company’s cost of capital, these may well be significantly distorted. Ironically, unadjusted IRRs are particularly treacherous because the reinvestment-rate distortion is most egregious precisely when managers tend to think their projects are most attractive. And since this amplification is not felt evenly across all projects, managers can’t simply correct for it by adjusting every IRR by a standard amount. (The amplification effect grows as a project’s fundamental health improves, as measured by NPV, and it varies depending on the unique timing of a project’s cash flows.)
How large is the potential impact of a flawed reinvestment-rate assumption? Managers at one large industrial company approved 23 major capital projects over five years on the basis of IRRs that averaged 77 percent. Recently, however, when we conducted an analysis with the reinvestment rate adjusted to the company’s cost of capital, the true average return fell to just 16 percent. The order of the most attractive projects also changed considerably. The top-ranked project based on IRR dropped to the tenth-most-attractive project. Most striking, the company’s highest-rated projects — showing IRRs of 800, 150, and 130 percent — dropped to just 15, 23, and 22 percent, respectively, once a realistic reinvestment rate was considered. Unfortunately, these investment decisions had already been made. Of course, IRRs this extreme are somewhat unusual. Yet even if a project’s IRR drops from 25 percent to 15 percent, the impact is considerable.