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FUTURE COMPUTING ENVIRONMENTS FOR MICROSIMULATION MODELING 148 a new generation of mixed microsimulation modeling systems that use both dynamic and static aging. Core attributes, for which sufficient dynamic knowledge exists, are aged dynamically. Noncore attributes are adjusted using appropriate cross-sectional joint distributions. CURRENT STATIC SOCIOECONOMIC MICRO ANALYTIC SIMULATION MODELS Overview During the past 25 years, socioeconomic microanalytic simulation models have been widely used to study the effects of alternative individual tax and transfer policies. The use of such models was encouraged by the relatively deterministic nature of the programs that implemented such policies.9 Static models have consistently been used to perform such studies. The choice is supported by the following observations: (1) the time horizon for such studies is generally relatively short; (2) the operating characteristics of greatest interest are detailed and deterministic in nature; and (3) the methods of growing the different dimensions of the micropopulation, although independent, are all intuitively straightforward and easily explained to policy makers. Two computer-based static modeling systems now being used to conduct such studies are SPSD/M in Canada and TRIM2 in the United States. In this section these two systems are described and compared. SPSD/M History The Social Policy Simulation Database/Model (SPSD/M) was developed in 1986 by the Social and Economic Studies Division of Statistics Canada. A beta test version of SPSD/M was completed in 1987, and the first publicly available version was completed in November 1988 (see Availability, Customer Base, and Technical Support below). Before SPSD/M existed, a few departments of the Canadian government had a virtual monopoly on the ability to do detailed analyses of the impacts of tax and transfer policy changes. These departments use models that were developed independently and that are substantially nonoverlapping in their capabilities. For example, the Department of Finance uses a microsimulation model that can recompute income tax liabilities for a sample of taxpayers, while the Department of Employment and Immigration has its own 9The implicit behavioral assumption underlying many such models is that individuals and tax and transfer units have knowledge of the programs that apply to them and will take maximum financial advantage of them. For some tax/transfer programs where it is known that all eligible units do not participate, a participation or takeup function is used to determine which units receive program benefits.