It is important to recognize the implications of the SIPP’s longitudinal design for the representativeness of the information that it collects. The SIPP sample is selected from the resident population of the United States, excluding persons living in military barracks or institutions. The sample is designed to be representative of this population at the time that it is drawn, and the initial respondents are weighted to Census Bureau estimates of the size of this population by age, sex, race, and Hispanic origin. The SIPP sample is dynamic, however. Over the life of a SIPP panel some respondents leave the sample and others are added. As a result, the sample size and its representativeness change over time.
Attrition. Respondents who refuse to continue participating in the survey, move to an unknown address, or move more than 100 miles from a SIPP primary sampling unit and cannot be interviewed by telephone are lost from the panel. Because they continue to belong to the population that the SIPP panel was selected to represent, the sample weights of other sample members must be adjusted to compensate for their loss. The 1992 panel had a 9.3 percent nonresponse rate to the initial interview and a cumulative nonresponse rate of 26.2 percent through the ninth interview.
Exits from the Population. Persons who die, move outside the country, enter institutions, or move into military barracks leave the population as well as the sample. Because these losses affect the population as well as the sample, they are not treated as attrition. There is no adjustment to the weights of other panel members to compensate for their loss.
Additions to the Sample. Persons who move into the households of panel members (including those who are born to panel members) become sample members and remain so for as long as they continue to reside with original panel members. Likewise, persons into whose household an adult SIPP panel member moves become sample members as well--again, for as along as the panel member continues to reside with them.
Births clearly represent additions to the population as well as the sample. Other persons added to the sample after the initial interview may or may not represent additions to the population that the SIPP sample represents. Persons who move into or return to the country, leave institutions, or move out of military barracks constitute additions to the population. If they move into SIPP households they become SIPP sample members, and through their addition to the sample the SIPP can be said to represent all persons who joined the population and moved into households that were included in the initial population. Persons who move into the population but form their own households cannot join the SIPP sample. Strictly speaking, then, the SIPP sample does not represent these additions to the population over time. But the SIPP sample weights, as we shall explain, take account of these additions, and so they are represented in number if not actual sample members.
SIPP Weights. To enable inferences from the SIPP sample to the total population the Census Bureau constructs both cross-sectional and longitudinal weights. The cross-sectional weights are created for each calendar month. Weights for a given month are assigned to all persons for whom data were collected in that month, and they are constructed so that they sum to an estimate of the total population by age, sex, race, and Hispanic origin in that month. These weights account for sample attrition (see below) as well as net additions to the population.
The Census Bureau assigns longitudinal weights to all initial sample members who remain through the final interview or leave the survey universe, providing that they miss no more than one consecutive interview.(1) For the 1992 panel, these persons constituted about 74 percent of the initial sample (where the latter includes first wave nonrespondents). These longitudinal weights are adjusted to compensate for persons who attrited through nonresponse (or were never interviewed), and at the outset they sum to the Census Bureau’s estimate of the SIPP population in March 1992. Because of panel members who exit the population as well as the sample, the weighted sample total declines over time. At any point after the first interview, the longitudinally-weighted SIPP panel represents the survivors of the population that the panel represented fully at the start.
The Census Bureau does not assign longitudinal weights to children born after the first interview. These children cannot be weighted with the same scheme that is used for sample members who were actually present for the initial interview, and the Bureau has elected not to apply an alternative weighting scheme.(2) After the first year, then, the weighted longitudinal sample contains no infants. A year later it contains no children under age two, and a year after that it contains no children under age three. For many research purposes--including ours-- this is not acceptable. Therefore, we have followed what has become a commonly used practice of assigning newborns the weights of their mothers.(3)
Adjustments for Nonresponse. Both the cross-sectional calendar month weights and the longitudinal weights take into account characteristics of the panel members who attrited, but the limitations of this adjustment must be recognized. The nonresponse adjustments cannot fully account for the ways in which the attriters may differ from panel members who remain in the sample because some of these differences cannot be known. For example, some of the attrition may be influenced by important changes in circumstances--loss of employment, divorce, birth of a child-- that occurred after the attriter’s last interview. In addition, attriters may simply be different in ways that are not observed but which affect their behavior post-attrition.
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