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ALTERNATIVE MODEL DESIGNS: PROGRAM PARTICIPATION FUNCTIONS AND THE ALLOCATION OF ANNUAL TO 105 MONTHLY VALUES IN TRIM2, MATH, AND HITSM The models generally derive participation rates for the baseline files that simulate current program law by comparing, within specified categories, the caseload counts from administrative records to the counts of simulated eligible units produced from the models themselves. Then they select eligible units to participate to reproduce the derived rates. Alternatively, they may, as in the TRIM2 AFDC participation routine, implement a participation equation, estimated with outside data, on the eligible population and then âcalibrateâ the equation so that the resulting caseload profile matches the administrative counts of key caseload characteristics. The degree to which the baseline files accord with program administrative data is viewed, by modelers and users alike, as critical to the credibility of the model. Moreover, the participation rates determined for the baseline files generally, either directly or indirectly, determine the participation patterns used in the simulation of program alternatives. The participation and calibration functions for TRIM2, MATH, and HITSM are described below for SSI, AFDC, and food stamps (i.e., in the order in which the models simulate the three programs). Supple mental Security Income Participation Functions TRIM2 The TRIM2 SSI participation function makes uses of information on reported participation in the March CPS, unlike the TRIM2 AFDC and food stamp functions. The SSI module sets up 25 categories of participants, by cross-tabulating five categories of benefit level by five unit types, including aged single persons, disabled single adults, aged couples, disabled couples, and disabled children. The module designates eligible people reporting SSI benefits as participants and compares their numbers in each of the 25 categories with target numbers of participants taken from the Social Security Bulletin Annual Statistical Supplement. Then, if the number reporting benefits is less than the target number of participants, the module selects at random additional eligible people not reporting SSI benefits to participate. The module assigns all designated participants the simulated SSI benefit value, regardless of whether they reported an amount to the CPS. Also, the module never allows CPS reporters who are simulated to be ineligible to participate. An additional step (which is optional) will align participation to state-specific control totals. This process is iterative, as participation status is first changed to move closer to state controls and then to satisfy the 25 benefit level and unit type controls. Comparison of the number of eligible and participating persons in a current law simulation produces implicit participation rates for each of the 25 categories. These participation rates may be used in simulations of program