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Documentation for Microsimulation Models: A Review of TRIM2, MATH, and HITSM
Kevin M.Hollenbeck
INTRODUCTION
As part of its investigation of microsimulation models, the Panel to Evaluate Microsimulation Models for Social Welfare Programs examined the extant documentation of three such models—TRIM2 (Transfer Income Model 2), MATH (Micro Analysis of Transfers to Households), and HITSM (Household Income and Tax Simulation Model). The precise documents examined are as follows: for TRIM2, Webb et al. (1982, 1986) and Bergsman (1989); for MATH, Doyle et al. (1989), Social & Scientific Systems, Inc. (n.d.), and Doyle (1989); and for HITSM, Lewin/ICF, Inc. (1988).
In some ways one might presume that the panel was on firmer ground in this examination than in other aspects of its investigation. After all, few members of the panel have actually performed an application with a microsimulation model. But with the widespread use of personal computers for word processing, spreadsheet applications, and information retrieval through database packages, millions of individuals in professional, technical, and clerical occupations, including all of the panel’s members, have navigated software documentation. Another common encounter with software documentation, particularly among
Kevin M.Hollenbeck is senior economist at the W.E.Upjohn Institute for Employment Research; he served as a member of the panel.
researchers and policy analysts, has come through use of statistical software packages.
Nevertheless, because of the way that microsimulation has developed and been applied, comparison of microsimulation model documentation to personal computer software or statistical package documentation is not necessarily appropriate. This is because microsimulation models have, for the most part, been marketed by their developers as a service rather than a good. The prototypical transaction for microsimulation involves a sponsor purchasing the services of an analyst who specifies model parameters and changes for an applications programmer, who runs the model. The model is being constantly updated and changed. An analogy might be the purchase of insurance from an agent who uses a computer model to examine outcomes of different premiums, investment returns, and life tables. The audience for the documentation of that model, just like the audience for the documentation of microsimulation models (up to this point), is limited to the purveyors of the service. The purchaser of the service is expected to have little interest in the technical details, only the outcomes.
From this perspective, then, the panel’s seeming ability to evaluate documentation was severely constrained. The role of the panel is analogous to providing advice to the purchaser of insurance (or sponsor of microsimulation) concerning documentation of the computer model used by the insurance company’s agent. In general, the panel’s belief is that the better the documentation, the better the model and thus the more comfortable the purchaser should be with the model’s outcomes.1
This chapter first establishes general criteria that can be used to evaluate documentation and then addresses the documents mentioned above for TRIM2, MATH, and HITSM. The final section evaluates all three sets of documentation based on an industry standard for software documentation.
General conclusions from this review of the documentation are as follows:
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In general, documentation can serve informational, instructional, or reference functions and must serve various audiences. Microsimulation model documentation in particular must cater to a wide range of backgrounds—from individuals with an interest in microsimulation but no technical expertise to policy analysts with technical expertise and programmers who might conduct an application or change a model in some manner. The TRIM2 documentation comes closest to serving these functions for all audiences; the HITSM documentation is intended primarily to provide information to a nontechnical audience.
