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DATABASES FOR MICROSIMULATION: A COMPARISON OF THE MARCH CPS AND SIPP 17 program provisions and analyzing the output by a variety of population characteristics AFDC, SSI, and food stamps serve relatively small fractions of the total U.S. population: about 10 percent of households receive food stamps, which is the most inclusive of the three programs. Hence, large samples are critical to permit the kinds of detailed analyses of these programs that decision makers want. â¢ Geographic identification by state The AFDC program, as noted above, is partly financed by the states, and benefit levels and other program features vary by state. Hence, it is critical for modeling AFDC that data records contain state identifiers. (The capability to model state differences in the AFDC program does not necessarily mean that there will beâor needs to beâa large enough sample size to produce reliable estimates by state.) â¢ Timeliness Policy makers typically want estimates for the effects of proposed changes in a program such as AFDC for a 5-year period starting when a change is implemented, which may be 1 or more years in the future. The availability of a survey microdata file inevitably lags behind the reference period of the information, given that the data must be recorded and processed before they can be released for public use. Hence, there is a premium on data sets that are updated frequently and processed in an expeditious manner. These data requirements for modeling the major income support programs on a cross-sectional basis do not take into account links to other kinds of programs such as job training or child support. In considering the attributes of a suitable database for modeling these programs, it is important to recognize the widening horizons of policy makers. There is growing interest in the dynamics of welfare program participation and particularly in how to encourage the transition from welfare to work. To model program dynamics requires monthly data on a longitudinal basis. As noted above, there is also more interest in program linkages, such as between child support and AFDC. These kinds of linkages require yet more data for input to models, such as data about the circumstances of noncustodial as well as custodial parents. SURVEY DESIGN OF THE MARCH CPS AND THE SIPP The CPS is a continuing cross-sectional survey of a sample of U.S. households that is conducted every month. Its primary purpose is to collect data on labor force status in the week prior to the survey for people aged 15 and older to permit determining the monthly unemployment rate for the nation and large states. (The survey provides annual average unemployment rates for all states.) In most months, the survey includes supplemental questions on other topics; for over 4 decades, the CPS has included questions on income and work experience during the previous calendar year, which now, as in many past years, constitute the March income supplement. (See Welniak  for a history of the March CPS. Income questions, which were first asked in 1947, were a supplement to
DATABASES FOR MICROSIMULATION: A COMPARISON OF THE MARCH CPS AND SIPP 18 the April questionnaire through 1955, while work experience questions were supplements to the February and April questionnaires through 1969.) The bulk of the funding for the CPS comes from the Bureau of Labor Statistics; the Census Bureau supports the March income supplement and some other supplements, while other agencies occasionally provide funding for special supplements. At present, the CPS sample includes about 60,000 housing units each month that are eligible for interview.2 In addition, the March supplement includes another 2,500 eligible housing units that had contained at least one adult of Hispanic origin as of the previous November interview, plus a small number of households of armed forces members living off post or with their families on post. The CPS sample design rotates housing units into and out of the survey, continually refreshing the sample with some new cases while retaining a large proportion of cases to reduce the variance of month-to-month estimates of change in unemployment. Each housing unit is in the sample for 4 months, out for 8 months, and in for another 4 months, so that each month one-eighth of the sample is new, three-eighths have been in the sample 1â3 months, and the remainder have been in the sample for 4â7 months (or, to compare successive March supplements: one-half of the sample in March of one year was in the sample in the preceding March). The interviews are conducted in person for the first month and then, to the extent possible, by telephone. Information is obtained for all of the residents found at each sample housing unit; adults provide information about children, and proxy responses are readily accepted for adults who do not respond for themselves. People who move into a sample housing unit are interviewed, but not people who leave. The Census Bureau makes no attempt at present to link the data across months for those household members who remain in the unit while it is in the sample.3 SIPP, in contrast, is a continuing panel survey of samples of the adult population aged 15 and older; adults provide information about children living with them. A new panel is started in February of each year, and the sample members are interviewed at 4-month intervals over a period of about 30 months. The SIPP is funded entirely by the Census Bureau, with oversight by a federal interagency committee chaired by the Office of Management and Budget. The primary purpose of the survey is to obtain information about the economic and social well-being of the population for use in policy analysis 2Additional addresses are canvassed each month but are dropped from the eligible total because the housing unit at the address is vacant or has been demolished, converted to nonresidential use, and so on. 3Researchers outside the Census Bureau have matched monthly CPS files, using exact-match techniques based on scrambled identifiers, and some of these matches were performed in order to estimate parameters for microsimulation models. For example, labor force transition probabilities in PRISM are based on matched CPS files, as are the transition probabilities in the Multi-Regional Policy Impact Simulation (MRPIS) model.
DATABASES FOR MICROSIMULATION: A COMPARISON OF THE MARCH CPS AND SIPP 19 (including modeling) and research. The questionnaire includes a core set of detailed questions about employment status, receipt of income, and program participation on a monthly basis. Each interview also generally includes one or more topical modules that are asked once or twice during the life of a panel. The initial sample size for the first 1984 SIPP panel was about 21,000 eligible households; initial sample sizes for the 1985 through 1989 panels were between 14,500 and 12,500 households; the initial sample size for the 1990 panel is about 21,500 households, while that for the 1991 panel is about 14,000 households. (Budget cuts necessitated repeated cutbacks in SIPP sample size and, in some cases, in the number of interviews for a panel. To fund the larger 1990 panel, the Census Bureau had to terminate the 1988 and 1989 panels at six and three interviews, respectively; see Bowie .) To date, most SIPP interviews (90â93%) have been conducted in person, with some telephone follow-up where feasible and appropriate in the judgment of the interviewer (proxy interviews are accepted when necessary). Sample members are followed through the life of the panel unless they leave the universe through death, emigration, or institutionalization or they move and cannot be traced. Children and adults who join the household of a sample member after the start of the panel are interviewed as long as they reside with a sample member. Both the March CPS and SIPP samples are designed to cover the population in the 50 states and the District of Columbia, excluding only inmates of institutions and those members of the armed forces living on post without their families. The SIPP does try to keep track of sample people aged 15 and older who move into such institutions as prisons and nursing homes and to bring them back into the survey if they return to a noninstitutional residence. The Census Bureau currently uses an integrated sample design for its major household surveys, including the CPS and SIPP and also the National Crime Survey and American Housing Survey. The surveys do not include the same households (except inadvertently), but the samples for each survey are drawn from similar geographic areas so as to minimize travel costs for interviewers and permit them to handle more than one survey. The survey designs and sampling frames are updated after each census. In the 1980s, the design for the 1984 SIPP panel and the March 1981 through March 1985 CPS is based on the 1970 census; the design for subsequent years is based on the 1980 census. The first stage in the sampling process for the CPS and SIPP is to divide the entire United States into primary sampling units (PSUs), comprising larger counties and independent cities and groups of smaller counties. The larger PSUs are selected with certainty for the samples; smaller PSUs are grouped into strata and subsampled. The CPS sample includes PSUs in every state and is designed to be state-representative (on an annual average basis for most states and a monthly basis for larger states); the SIPP sample, being much smaller,
DATABASES FOR MICROSIMULATION: A COMPARISON OF THE MARCH CPS AND SIPP 20 does not generally share these features (although the 1984 panel had PSUs in every state). The final stages in the sampling process are to obtain addresses in each sampled PSU and select clusters of two to four households for interviewing. The addresses represent a combination of decennial census addresses and addressess that are obtained through field canvass, the latter principally in areas of new housing construction. The CPS oversamples smaller states (see above), but, otherwise, there is no oversampling of particular population groups in the CPS or SIPP with two exceptions. As noted above, the March CPS includes a Hispanic supplement identified from an earlier interview. The 1990 SIPP panel includes about 3,500 additional cases of households continued from the 1989 panel, selected because they were headed by blacks, Hispanics, or single female parents at the first wave of the 1989 panel. The Census Bureau is currently planning to redesign the CPS and SIPP samples (and those for other surveys) based on the results of the 1990 census. An original goal for the CPS redesign was to effect a major sample expansion, so that all 50 states and the District of Columbia would have reliable labor force data on a monthly basis. The proposal was to implement a two-phase interviewing cycle each month, whereby national estimates would be based on a sample interviewed in the course of one week each month, and state estimates would be based on the interviews in week one plus interviews conducted with additional sample cases in week two (both samples would have the same reference week). However, the administration was not successful in obtaining fiscal year 1991 funding to plan for expansion of the CPS sample; hence, work has stopped. However, the concept could be revived at a later date. A current goal for the SIPP redesign is to oversample low-income households that are identified in the 1990 census, but in a manner that does not adversely affect the sample sizes for people aged 55 and older. The redesigned SIPP sample will also likely eliminate clustering in the final stage of sample selection. (See Waite et al.  for information on the CPS and SIPP sample redesign plans.) The newly designed samples for CPS and SIPP will be implemented around 1994â1995. At that time, other design changes that are deemed necessary and appropriate will be introduced as well. For the CPS, in addition to the sample redesign based on the census, the Census Bureau plans to introduce an improved labor force questionnaire (which will have been tested on a supplemental sample of 15,000 households in 1992â1993). The Census Bureau also hopes to convert all CPS data collection to computer-assisted telephone and personal interviewing (CATI/CAPI) and to complete a new processing system that will take advantage of the quasi- longitudinal nature of the survey to improve the quality of each month's data by using information from prior months (see Butz and Plewes, 1990). With regard to the SIPP, the Census Bureau is currently examining all aspects of the surveyâquestionnaire design and content, panel design, data