Adults followed in the SIPP panel come from a nationally representative sample of households in the civilian, noninstitutionalized population of the United States. Sample members were interviewed once every four months during the 48-month panel period. If original (primary) sample members older than age 14 moved from their original residences, they were interviewed at their new addresses. Secondary sample members--those who were not part of the original sample but who lived with primary sample members after the first interview--were interviewed if they were in the same household as primary sample members.
The Census Bureau used multistage sampling techniques to select a representative set of households for the 1996 SIPP panels. The first interviews for the panel began in April 1996 with a sample of 40,188 households and 95,402 primary sample members, where households in the low-income stratum were sampled at 1.66 times the rate of the higher-income stratum. (1) Sample households were divided into four "rotation groups" of roughly equal size, and one rotation group was interviewed each month. Thus, each household was interviewed in four-month intervals, called "waves." The 1996 SIPP contains 12 waves, which provide 48 months of data for each person in the sample.(2)
At each interview, sample members provided information about their experiences during the preceding four-month period, called the "reference period." For example, people in rotation group 1 whose wave 1 interviews were conducted in April 1996 (the earliest interviews) were asked about their experiences between December 1995 and March 1996. Similarly, people in rotation group 4 whose wave 12 interviews were conducted in March 2000 (the latest interviews) were asked about their experiences between November 1999 and February 2000. Thus, the 12 reference periods for the 1996 SIPP panel cover December 1995 through February 2000.
The 1996 SIPP interviews were administered using computer-assisted interviewing (CAI) to increase data quality. CAI, used for the first time in the 1996 SIPP, permitted automatic consistency checks of reported data during the interview and allowed for the use of prior-wave data for editing missing data.
The SIPP questionnaire is made up of the core questions and the topical modules. The core questions provide information on (1) demographic characteristics; and (2) work behavior, income, and program participation for each of the four months preceding the interview date. The core questions were asked in every wave interview. Sample members were asked the topical module questions after the core questions. The content of the topical module changed from wave to wave. For our purposes, the topical modules administered in wave 1 are of special interest, because they contain information on respondents' prepanel experiences (see Section 4 below).