Adjustments within discount rates to cater for uncertainty—Guidelines

Deterministic discounted cash flow (DCF) analysis is a well-accepted technique in engineering appraisals. Common practice is to incorporate all uncertainty influences within a single variable—namely, the discount rate—which also represents the time value of money. Commentary already exists in the literature that such a practice is expedient but not rational and has shortcomings. This article examines the error involved in this practice and provides guidelines and precautions for using blanket or constant discount rates in dealing with uncertainties. It shows the adjustments necessary for any given investment scenario. This is done through establishing equivalence of the expected utility of deterministic and probabilistic present worth, allowing a rate adjustment to be calculated. Numerical studies look at the relationship or trends of this rate adjustment to the key analysis variables. Generally, it is found that the rate adjustment should be decreased as the timing of a cash flow's occurrence increases, increased as the variance of the cash flow increases, kept almost as an additive constant as the base rate increases, and increased as the investor's level of risk aversion increases. The article provides practitioner-friendly usable guidelines for adjusting rates, something that is unavailable elsewhere in the literature.