Every unit of real estate is unique in terms of its location, the building, and its financing. This makes pricing difficult, increases search costs, creates information asymmetry, and greatly restricts substitutability. To get around this problem, economists, beginning with Muth (1960), define supply in terms of service units; that is, any physical unit can be deconstructed into the services that it provides. Olsen (1969) describes these units of housing services as an unobservable theoretical construct. Housing stock depreciates, making it qualitatively different from new buildings. The market-equilibrating process operates across multiple quality levels. Further, the real estate market is typically divided into residential, commercial, and industrial segments. It can also be further divided into subcategories like recreational, income-generating, historical or protected, and the like.