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INTRODUCTION 4 Chapter 8, by Cohen, Billard, Betson, and Ericksen, documents and discusses the results of a validation experiment that the panel conducted with the TRIM2 model. The experiment combined a sensitivity analysis, which varied three TRIM2 components, with an external validation study. The three components were the input database, namely, whether or not the records were adjusted for population undercount; the routines used to age the database; and the routines used to convert annual income and employment variables in the March CPS to monthly values. External validity was assessed by having the model use a 1983 database to simulate AFDC and supplemental security income (SSI) program law in 1987 and then comparing the results with administrative data on AFDC recipients for 1987 from the Integrated Quality Control System (IQCS). In Chapter 9 Grummer-Strawn and Espenshade review the literature on evaluations of the quality of projections of the U.S. population produced by the Census Bureau and the Social Security Actuary through the use of cell-based models. These projections are important to assess because they are used by many microsimulation models as controls in the database aging process. In addition, the authors' discussion of the experience in validating these types of projections offers a useful contrast to the experience in microsimulation model validation discussed in Chapters 7 and 8. Model Documentation Good documentation is essential for people who want to use or develop a model and for people who want to understand the model outputs. In Chapter 10 Hollenbeck provides a critical review of the extant documentation of TRIM2, MATH, and HITSM. Hollenbeck compares the documentation for these models with a software documentation standard developed by the Institute of Electrical and Electronics Engineers, Inc. MODELS Seven major microsimulation models are discussed in one or more of the chapters. See the Appendix to Part II of Volume I for more detailed descriptions; see also Lewis and Michel (1990) for a recent review of the use of microsimulation modeling in tax and transfer policy analysis, including histories of the development of TRIM2, DYNASIM2, and PRISM. Dynamic Simulation of Income Model 2 (DYNASIM2) DYNASIM2 is a dynamic model of demographic and labor force processes and public and private retirement income programs, including social security, employer pensions, SSI, and individual retirement accounts. The model also