Capital Budgeting: NPV, IRR, and MIRR Compared
This paper examines capital budgeting techniques — specifically Net Present Value (NPV), Internal Rate of Return (IRR), and Modified Internal Rate of Return (MIRR) — with a focus on the reinvestment rate assumptions embedded in each method. It argues that using the firm's cost of capital as the reinvestment rate is problematic because it ignores project-specific risk and treats reinvestment as incremental to the project being evaluated. The paper then explains why NPV is considered the most theoretically sound method, particularly when choosing among mutually exclusive projects with finite resources. Finally, it addresses why managers frequently favor simpler methods like payback period and IRR in practice, despite their theoretical shortcomings.
- Reinvestment Rate Assumptions in NPV, IRR, and MIRR: Cost of capital as shared reinvestment rate assumption
- Problems with the MIRR Reinvestment Assumption: Project-specific risk and reinvestment irrelevance flaws
- Why NPV Is the Most Theoretically Sound Method: Dollar value superiority over IRR and payback period
- Why Managers Prefer IRR and Payback Period in Practice: Managerial preference for simpler, easier-to-sell methods
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What makes this paper effective
- Clearly distinguishes between the theoretical ideal (NPV) and common practice (IRR, payback period), grounding the contrast in realistic managerial behavior.
- Identifies two distinct reasons why the reinvestment rate assumption is problematic — project-specific risk and the irrelevance of post-project reinvestment — rather than treating them as a single issue.
- Uses concise, direct prose appropriate for a finance audience, avoiding unnecessary jargon while maintaining technical accuracy.
Key academic technique demonstrated
The paper demonstrates comparative analysis of financial methods: it evaluates NPV, IRR, and MIRR against a consistent criterion (reinvestment rate assumptions) before pivoting to a second criterion (dollar value vs. percentage return) to build a cumulative case for NPV's superiority. This layered argumentation — technical critique followed by practical context — is a hallmark of applied finance writing.
Structure breakdown
The paper opens by identifying the shared reinvestment rate assumption across all three methods, then drills into the specific flaws of MIRR and IRR assumptions. It next makes a positive case for NPV on both theoretical and practical grounds, including a comparison with payback period. The final section shifts to behavioral finance, explaining the gap between best practice and managerial preference. The structure moves logically from critique to recommendation to real-world qualification.
Reinvestment Rate Assumptions in NPV, IRR, and MIRR
Reinvestment rates are an embedded assumption in the NPV, IRR, and MIRR methods. In each of these methods, the firm's cost of capital is typically used as the discount rate. The cost of capital is comprised of the different elements of the capital structure, and in each case the reinvestment rate is a key factor. It is assumed that the cost of capital equals the reinvestment rate under each of these methods, and this assumption introduces the potential for error.
Problems with the MIRR Reinvestment Assumption
The assumed reinvestment rate of MIRR is the cost of capital, but this is problematic for a couple of reasons. The first is that it does not take into account project-specific risk (Damodar, n.d.). Each project carries its own risk profile. Thus, the reinvestment rate should not necessarily be the same rate used in an NPV, IRR, or MIRR calculation. In reality, the reinvestment rate of return could differ substantially. The firm's cost of capital may change over time, or a given project may vary significantly from the firm's normal business activities and therefore carry a meaningfully different risk characteristic.
The second reason why this assumption is problematic is that reinvestment is not ultimately relevant to the project itself. Once capital is returned from the project, any subsequent reinvestment would be subject to its own NPV, IRR, or MIRR calculation. Reinvestment should not be considered incremental to the project at hand, unless the funds are to be reinvested directly back into that same project.
The assumed return under IRR — which is the IRR rate itself — is similarly problematic, as is the assumption embedded in NPV. The problems are analogous: a discrepancy can exist between the assumed rate and the actual rate at which money is reinvested, and that discrepancy can skew a project's apparent value when performing these calculations.
Why NPV Is the Most Theoretically Sound Method
Net present value is regarded as the most theoretically sound capital budgeting technique for several reasons. The base calculation underlying NPV and IRR is the same; the key difference is that IRR expresses return as a percentage while NPV expresses it as a dollar value. The point at which the IRR exceeds the discount rate is also the point at which the NPV becomes positive. However, NPV takes into account the dollar value of the transaction, making it more robust for decision-making because companies typically have finite resources.
When choosing between mutually exclusive projects, it is important to select the project that returns the greatest dollar value to shareholders. If a project with a higher IRR is chosen but it generates a lower dollar value — leaving some capital on the sidelines — the opportunity cost of those idle funds must be considered. NPV captures this trade-off directly, which is precisely why it is superior to IRR in such comparisons.
While MIRR offers some improvement over IRR by assuming reinvestment at the cost of capital rather than the IRR rate itself, this does not change the fundamental relationship between MIRR and NPV. NPV remains superior for the same reasons it is better than IRR. It should also be noted that NPV is superior to other methods such as the payback period, because the payback period method ignores cash flows that occur after payback — flows that can sometimes be substantial.
References
"Chapter 10: Capital budgeting — why it matters" (2014) [University]. In possession of the author.
Damodar, A. (n.d.). The components of risk. NYU Stern School of Business. Retrieved July 12, 2016, from http://pages.stern.nyu.edu/~adamodar/New_Home_Page/invfables/riskcomponents.htm
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