Double vision: Insights about the origin and interpretation of multiple IRRs

The ability of a project's internal rate of return (IRR) to quantify its economic return has been questioned by many scholars over the past 60 years, most recently by Magni (2010 Magni, C.A. (2010) Average internal rate of return and investment decisions: a new perspective. The Engineering Economist, 55(2), 150–180. Google Scholar], 2013 Magni, C.A. (2013) The internal rate of return approach and the AIRR paradigm: a refutation and a corroboration. The Engineering Economist, 58(2), 73–111. Google Scholar]). Although IRR is a plausible—albeit imperfect—measure of a project's economic return when the cash flow stream is conventional, IRR can be an untenable measure of an unconventional project's economic return. The goal of this article is to identify a simple, intuitive explanation of IRR, one that can be applied to any cash flow pattern. To do this, the article shows how a project's IRR systematically changes when it first crosses from the conventional into the unconventional realm (i.e., a small cash outflow is appended to a conventional cash flow stream) and then as it becomes progressively more unconventional. This process reveals that the most robust economic interpretation of IRR—for both conventional and unconventional projects—is that a project's IRRs are external benchmarks that divide the set of all plausible discount rates into positive and negative net present value (NPV) ranges, rather than internally generated returns. Because it can be difficult to estimate a project's cost of capital with precision, this information can help guide the sensitivity analysis of a project.