A new method for calculating reliable discounts for lack of marketability (DLOMs) for Limited Partner (i.e., non-controlling) family limited partnership (FLP) interests, which we term the Managed Asset Portfolio Market (MAPM) Analysis, is developed. DLOMs typically are the largest valuation adjustment in, and often the most contentious part of, appraisals of closely-held company interests, including FLPs. U.S. Courts (and SEC) require appraisal experts to calculate objective, market-based DLOMs (that satisfy the Courts’ “gatekeeping” test to be qualified as expert testimony in litigation). Appraisals of market value are rooted in Economics and, as such, are inseparably linked to an underlying analysis of supply and demand. Although economic theory and principles are not absent in business valuation theory and practice, a market analysis often is only an implicit, and at times an unrecognizable, part of them. The MAPM Analysis is explicitly built upon an underlying economic foundation of market supply and demand, which, in conjunction with publicly-traded market data, provides a relevant and reliable market basis for estimating DLOMs for FLPs.