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DATABASES FOR MICROSIMULATION: A COMPARISON OF THE MARCH CPS AND SIPP 43 this detail on the public-use tape. Hence, the income-support program models, which need more income detail than provided on the CPS prior to March 1989, had to allocate the totals to the specific sources by using information provided on recipiency of each source. In contrast, SIPP, from the beginning, has provided intrayear detail on about 50 separate unearned income sources, plus components of earnings. House hold Composition Reference Pe riod The March CPS ascertains prior-year income and employment for the household members present at the time of the interview. Hence, the survey omits income received during the preceding year by persons who died, emigrated, or entered institutions before the interview. Moreover, the survey ignores changes in household composition during the year and thereby portrays inaccurately the economic well-being of many people. For example, a poor female-headed family may have been better off during the income reference year if the husband had been present for all or part of the year. At present, the income-support program models based on the CPS do not attempt to address this problem. In contrast, SIPP keeps tracks of household composition changes on a month-by-month basis, just as income and other variables are recorded on a monthly basis. For a model based on SIPP cross-sectional data for, say, a month or a wave, there will be few problems of mismatched households and income or other characteristics. The issue becomes more complex for models that attempt to use the longitudinal SIPP data over time. Despite considerable research on the subject, it is not clear how best to define households, families, or tax or transfer program filing units to take account of changing composition across time (see, e.g., Citro, Hernandez, and Moorman, 1986). Nonetheless, SIPP, unlike the CPS, affords the capability for appropriate measurement of household composition changes. House holds and Familie s Versus Program Filing Units The March CPS identifies households, primary families, and subfamilies. The data do not identify filing units, although income support programs typically exclude one or more household members in determining eligibility for benefits. Similarly, tax filing units may exclude some household members (and include others who are not resident in the household). A major task performed by the income-support program models in processing the CPS input data is to create status definer variables and filing units of all possible types. However, the CPS does not provide all of the information necessary for accurate identification of program units: for example, food stamp units that do not comprise the entire household are often difficult to identify. The importance of accurate measurement of household composition and