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Suggested Citation:"Critique." National Research Council. 1991. Improving Information for Social Policy Decisions -- The Uses of Microsimulation Modeling: Volume II, Technical Papers. Washington, DC: The National Academies Press. doi: 10.17226/1853.
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Page 336
Suggested Citation:"Critique." National Research Council. 1991. Improving Information for Social Policy Decisions -- The Uses of Microsimulation Modeling: Volume II, Technical Papers. Washington, DC: The National Academies Press. doi: 10.17226/1853.
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Page 337

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DOCUMENTATION FOR MICROSIMULATION MODELS: A REVIEW OF TRIM2, MATH, AND HITSM 336 standard for software documentation. (A later section details this standard and evaluates the microsimulation model documentation with respect to it.) With these criteria in mind, the next three sections review each model's documentation. Each section provides a summary of the format and purpose of the documentation, a critique of the documentation, and suggestions for improvement. TRIM2 The TRIM2 documentation is comprised of three parts—the Reference Manual (Webb et al., 1986); Simulation Modules (Webb et al., 1982); and Codebook (Bergsman, 1989). The Simulation Modules is a two- volume document, and presumably there are multiple volumes of codebooks, although the panel had only a single document.2 The Reference Manual (Webb et al., 1986:15) itself explains the purpose of the three parts: The TRIM2 Reference Manual is the overall reference for the TRIM2 framework for microsimulation…. Chapter I is an introduction to TRIM2 for anyone who is not acquainted with [it]. Chapter II is a tutorial for anyone who will be using TRIM2 directly or indirectly. Chapter III is a reference for persons actually submitting TRIM2 runs. Chapters VI, X, XII, and XIII and Appendices A and C will be used by programmers adding or modifying simulation modules. The remaining chapters will be used primarily by TRIM2 system programmers. TRIM2 Simulation Modules occupies two or more looseleaf volumes and contains a chapter for each simulation module…. Each chapter contains a text description and technical specification of the module, a description of each subroutine, and definitions and values for all parameters. TRIM2 Codebook contains definitions of all variables used by TRIM2 and a catalog of frequently used household micro files. Critique This section attempts to point out the strengths and weaknesses in the TRIM2 documentation. By a number of standards, the TRIM2 documentation dominates that of the other two models. This model is portable, and the documentation would be useful to individuals who need to learn how to run the model, to analysts, and especially to programmers, who will change and update it. 2According to the Urban Institute staff, until 1990, there has been only one codebook volume. Each year, the codebook is updated to include new variables, clarifications about variable definitions, marginal statistics, and so on. Because a main goal of the conversion process is to create a standard file each year, it has not been necessary to have different codebooks for different years. In 1990, a new codebook was created because of the major structural differences between the March 1989 CPS and earlier files. The annual codebook update will now be applied to the “March 1989 CPS and later” codebook.

DOCUMENTATION FOR MICROSIMULATION MODELS: A REVIEW OF TRIM2, MATH, AND HITSM 337 TRIM2's Reference Manual suggests that Chapter I was designed for individuals unfamiliar with the model. In fact, Chapter I packs into seven pages of text a discussion about the differences between discrete and continuous simulation models, a discussion of the differences between static and dynamic aging, a claim that TRIM2 could handle microsimulation of corporations, a history of the development of TRIM2, a discussion of the (very) technical advantages of TRIM2 over previous versions, descriptions of all the major simulation modules in TRIM2, a description of the documentation, and the model's computer requirements. In short, the chapter fails because it is an amalgam of terse discussions of widely different subjects and does not provide an overview of the model for those unfamiliar with it. Perhaps the biggest problem with the TRIM2 documentation is that its updates are not integrated so that the document maintains consistency. Chapter I is a prime example. A government sponsor or other potential user of the model who might focus on this chapter would probably like to know, for example, whether it captures changes in federal income taxes instituted by the 1986 Tax Reform Act, if the JOBS (Job Opportunities and Basic Skills) program is simulated in the AFDC (Aid to Families with Dependent Children) module (which a potential sponsor might have heard was the model's comparative advantage), or whether recent changes in the food stamp program are simulated. The reader of the chapter would get no clue. Furthermore, the chapter indicates that the current reference codebook for TRIM2 variables is the March 1980 Current Population Survey (CPS) TRIM codebook. Chapter VI presents another example of inconsistent cross-references. Section C of the chapter refers to a document called the TRIM2 Master Routine Manual. The context suggests that this reference, which is repeated several times, should be to the Simulation Modules document. Chapter II of the Reference Manual is actually a tutorial intended to teach the reader exactly how to run the model. The chapter is well organized and very readable. It has ma ny examples of run setups and sample output. This is an excellent chapter that gives the reader the sense that TRIM2 is a portable model. The remainder of the Reference Manual contains technical details aimed primarily at programmers. The subjects covered are job control language (JCL) set-up, the Central TRIM2 Directory, variable naming conventions, programming standards, utilities, and so forth. It was impossible to evaluate the accuracy of these chapters; however, they seem to be clear, consistently formatted, and complete. One bothersome detail about the Reference Manual, however, is that some rather arbitrary decisions seem to have been made about what to include vis-à-vis the Simulation Modules and the Codebook. For example, the documentation for input and output modules, RDFILE and WRFILE, is included in the Reference Manual. However, XPORT, which is an optional type of output, and SIMTAB,

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This volume, second in the series, provides essential background material for policy analysts, researchers, statisticians, and others interested in the application of microsimulation techniques to develop estimates of the costs and population impacts of proposed changes in government policies ranging from welfare to retirement income to health care to taxes.

The material spans data inputs to models, design and computer implementation of models, validation of model outputs, and model documentation.

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