SIPP is administered in panels and conducted in waves and rotation groups. Within a SIPP panel, the entire sample is interviewed at 4-month intervals. These groups of interviews are called waves. The first time an interviewer contacts a household, for example, is Wave 1; the second time is Wave 2, and so forth. As discussed in Chapter 3 of the SIPP Users' Guide, each wave contains core questions that are asked each time, along with topical questions that vary from one wave to the next.
Sample members within each panel are divided into four subsamples of roughly equal size; each subsample is referred to as a rotation group. One rotation group is interviewed each month.1 During the interview, information is collected about the previous 4 months, which are referred to as reference months. Thus, each sample member is interviewed every 4 months, with information about the previous 4-month period collected in each interview (see Table 2-2).
The original design of SIPP called for an initial selection of a nationally representative sample of households, with all adults in those households being interviewed once every 4 months over a 32-month period. In addition, interviews were to be conducted with any other adults living with original sample members at subsequent waves. The first sample, the 1984 Panel, began interviews in October 1983. The 1985 Panel began in February 1985. Subsequent panels began in February of each calendar year, resulting in concurrent administration of the survey in multiple panels. Because of budget constraints, actual panel duration has varied. The original goal was to have panels covering eight waves (32 months). In several instances, panels were terminated after seven waves (28 months). Two panels were terminated even earlier: 1988 (six waves) and 1989 (three waves).
With certain exceptions (Table 2-1), each panel overlapped part of the previous panel, with the result that there were two or three active panels at any given time. The overlap allows analysts to combine records from different panels, thus having larger samples (and lower standard errors) for cross-sectional analyses.2 The overlapping feature of the SIPP design was dropped with the 1996 redesign. Standard errors have remained small since the redesign because the 1996 and following panels each have target sample sizes of at least 37,000 interviewed households for Wave 1, almost twice the size of two of the previous panels.
|1984||Oct. 83||Jul. 86||20,897||55,400||9||2, 8|
|1985||Feb. 85||Aug. 87||14,306||37,800||8||2|
|1986||Feb. 86||Apr. 88||12,425||32,800||7||3|
|1987||Feb. 87||May 89||12,527||33,100||7|||
|1988||Feb. 88||Jan. 90||12,725||33,500||6|
|1989||Feb. 89||Jan. 90||12,867||33,800||3|
|1990||Feb. 90||Sep. 92||23,627||61,900||8|
|1991||Feb. 91||Sep. 93||15,626||40,800||8|
|1992||Feb. 92||May 95||21,577||56,300||10|||
|1993||Feb. 93||Jan. 96||21,823||56,800||9|
|1996||Apr. 96||Mar. 00||40,188||95,402||13|
Although most available data predate the 1996 redesign (discussed in Chapter 1 of the SIPP Users' Guide), the redesign affected the nature of some panels. In preparation for the redesign, the Census Bureau canceled the 1994 and 1995 Panels and extended the 1992 and 1993 Panels (Table 2-1). The last 1993 Panel interview took place in January 1996 to ensure that data would remain continuous. Also in 1996, the Census Bureau initiated the Survey of Program Dynamics (SPD) as an extension of SIPP. For the SPD, the Census Bureau began recontacting people in the 1992 and 1993 SIPP panels and will continue annual data collection through 2002. The plan is to yield 10 years of data (1992.2001) for those two panels to support analyses of changes during welfare reform and for the pre- and postreform periods (Chapter 1 of the SIPP Users' Guide).
One full 4-month cycle of administering the questionnaire to the entire panel is a wave. The 1984 through 1993 Panels were designed to have eight waves each, although more often than not the number of waves actually administered was different (Table 2-1). The 1996 Panel has 12 waves. Rotation groups are random subsamples of approximately equal size. Each month, the members of one rotation group are interviewed; over the course of 4 months, all rotation groups are interviewed, providing data for the full set of 4 months. For many survey items, SIPP collects data for each of the 4 calendar months preceding the interview month. Those 4 months together are called reference months, or the reference period. (Table 2-2 provides an illustration of the reference months for the various rotation groups in each wave of the 1996 Panel.)
The reference period length and the timing of the interviews address several concerns: respondent recall error, which increases as the recall period lengthens; respondent burden, which increases with the number of times they are interviewed; and the costs of frequent interviews. By spreading the interviews for each wave evenly over 4 months, the rotation group structure allows the Census Bureau to keep a skilled and experienced team of interviewers in the field year round. This eases management burden and allows Census Bureau interviewers to master the complexities of the SIPP questionnaire and to maintain that mastery.
Each SIPP panel prior to 1990 had fewer than eight waves or contained one wave that consisted of fewer than four rotation groups (Table 2-1). As discussed in Chapter 3 of the SIPP Users' Guide, the questionnaire administered at each wave contains core questions, those asked at every interview, along with sections containing topical questions that vary from one wave to the next. Respondents in the skipped rotation groups have no gap in core data, but they do not provide core data for the full duration of the panel, and they lack topical data for the wave in which they were skipped. Analysts should be alert to the consequences of the skipped rotations: some topical information is not available for the full sample, and the length of time an analyst can follow adults from the original sample is reduced for selected rotation groups.
The reference period for most core items is the 4-month period preceding the month of the interview for the given wave. Data for most core items are collected for each of the preceding 4 months. Some data on labor force characteristics are collected with weekly resolution. Subsequently, weekly labor force characteristics are recorded on a monthly basis.
After the basic demographic information, one of the first items in the SIPP interview illustrates the availability of time-specific data in SIPP. The respondent is asked if he or she had a health insurance plan at any time during the previous 4 months. If the answer is yes, SIPP asks if the respondent had coverage in each of the individual 4 months. Thus data are collected for 4 individual months at each wave. Over the course of a 13-wave panel, data are collected for 52 consecutive months for each panel member. For the 1996 Panel, the rotation groups were interviewed in order. Specifically, for Wave 1, rotation group 1 was interviewed in April, rotation group 2 in May, rotation group 3 in June, and rotation group 4 in July. For previous panels, however, the specific months varied slightly among rotation groups. With the 1990 Panel, for instance, panel members in rotation group 2 were interviewed first; rotation group 1 was actually the fourth rotation group surveyed in that panel.3
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