The weight for a responding unit in a survey data set is an estimate of the number of units in the target population that the responding unit represents. In general, since population units may be sampled with different selection probabilities and since response rates and coverage rates may vary across subpopulations, different responding units represent different numbers of units in the population. The use of weights in survey analysis compensates for this differential representation, thus producing estimates that relate to the target population.
Most SIPP panels have not sampled different subpopulations at different rates (the exceptions are the 1990 and 1996 Panels). However, there are some minor variations in sampling rates in all SIPP panels and, more important, there are appreciable variations in response and coverage rates across subpopulations. As a result, there is nontrivial variation in SIPP weights (see SIPP Quality Profile, 3rd Ed. [U.S. Census Bureau, 1998a, Table 8.1]). For example, in Wave 1 of the 1993 Panel, the final person lower quartile weight is 4,400 and the upper quartile weight is 5,245 (the maximum weight is 28,695). A respondent with a final person weight of 4,400 represents 4,400 people in the U.S. population for the reference month, whereas a respondent with a weight of 5,245 represents 5,245 people. Because weights in SIPP vary over a sufficiently large range of values, performing unweighted analyses may produce appreciably biased estimates for the U.S. population.
Table 8-1 illustrates the effects of weighting on a selection of estimates obtained from Wave 1 of the 1990 Panel. The 1990 Panel included an oversample of households headed by blacks, Hispanics, and females with no spouse present and living with relatives. Since those groups are overrepresented in this sample, failure to use the weights would lead to overrepresentation of the groups in the population estimates based on that sample. At the household level, the unweighted percentage of households headed by females with no spouse present is 14.3 percent, whereas the weighted estimate is 11.7 percent. At the person level, the magnitude of the differences between weighted and unweighted estimates is less, but still appreciable.
|Female-headed households with no spouse present, living with relatives||11.7||14.3|
|American Indian, Eskimo, or Aleut||0.6||0.6|
|Asian or Pacific Islanders||2.9||2.9|
|Age over 65 years||10.4||10.6|
|Receiving Food Stamps [RCUTYP27 (FOODSTMP)]||6.7||7.7|
Weights Available in SIPP Files
Table 8-2 lists the weight variables in SIPP data files for the 1996 and 1990.1993 Panels. For earlier panels, the user should refer to the data dictionary for the particular file.
|Core Wave Files|
|WPFINWGT (FNLWGT)||Reference month, final weight of person|
|WHFNWGT (HWGT)||Reference month, final weight of household|
|WFFINWGT (FWGT)||Reference month, final weight of family|
|WSFINWGT (SWGT)||Reference month, final weight of related subfamily|
|WPFINWGT (P5WGT)a||Interview (5th) month, final weight of person|
|WHFNWGT (H5WGT)a||Interview (5th) month, final weight of household|
|Topical Module Files|
|WPFINWGT (FINALWGT)||Prior to 1996: interview month, final weight of person. 1996+: 4th reference month, final weight of person|
|Full Panel Filesb|
|WPFINWGT (FNLWGT)x||Calendar year x, final weight of people in the calendar year cohort|
|PNLWGT (Not kept for 1996 panel)||Final weight for people in full panel cohort|
Choosing a Weight
The decision of which weight to use for a given analysis depends on the population of interest for that analysis. Useful guidance for choosing the correct set of weights is to consider to what population the results are intended to apply.
The weights in the SIPP files are constructed for sample cohorts defined by:
Users can choose to base their analyses on:
Monthly (cross-sectional) weights allow the use of all available data for a given month. For this type of analysis, users can choose among the following units of analysis:
Analysts can use longitudinal samples to follow the same people over time and hence study such issues as the dynamics of program participation, lengths of poverty spells, and changes in other circumstances (e.g., household composition). The longitudinal weights allow the inclusion of all people for whom data were collected for every month of the period involved (calendar year or full panel period), including those who left the target population through death or because they moved to an ineligible address (institution, foreign living quarters, military barracks), as well as those for whom data were imputed for missing months. The Census Bureau makes nonresponse adjustments to the longitudinal weights to compensate for panel attrition and poststratification adjustments to make the weighted sample totals conform to population totals for key variables.
How Weights Are Constructed
This section describes how the weights are constructed. The basic components for all the different sets of weights are the same, namely:
Reference month final weights are provided on the SIPP core wave files for persons, households, families, and subfamilies; interview month final weights are provided for persons and households. The special weights for persons are constructed first. The household, family, and related subfamily final weights are derived from the final person weights. This section summarizes the steps involved in constructing the various sets of weights, starting with the final person weights for a reference or interview month. Appendix C provides the technical details and reasons for some of the adjustments.
The reference and interview month weights1 for people on the core wave files are computed (i.e., are nonzero) for all responding sample members who are .in scope. (i.e., a part of the survey's universe.the resident, noninstitutional population of the United States) in the specified month.2 A number of factors lead to fluctuations in sample size from month to month. They include births, deaths, immigration, and emigration from the population (and therefore from the sample). In addition to those population dynamics, people move into and out of the sample as a result of the changing household composition of sample members. (Chapter 2 describes the SIPP "following rules".)
In Wave 1, the weight for each sample person per month is a product of four components:
In subsequent waves, each person receives an initial weight that is carried over from the preceding wave. This weight is adjusted to compensate for changes in the sample between waves resulting from movers and nonresponse, and then it is realigned to match the population totals for the reference or interview month:
Final Full Panel and Calendar Year Weights
Final full panel and final calendar year weights are provided on the full panel files for eligible sample members. There is one set of final panel weights and generally more than one set of calendar year weights, one for each calendar year covered by the panel. The 1992 Panel file has three sets of calendar year weights because that panel covered 3 calendar years. The 1996 Panel file will have four sets of calendar year weights.
Final panel weights are computed only for people who are in the sample at Wave 1 of the panel and for whom data are obtained (either reported or imputed) for every month of the panel for which they were in scope for the survey. Other people in the panel file are assigned weights of zero. Most people with nonzero final panel weights have provided data for all months of the panel. However, people who missed a wave and whose missing wave data were imputed and people who provided data up to the point that they left the survey (through death or because they moved to an ineligible address) are also assigned nonzero final panel weights. (In core panels, it also includes those missing up to two consecutive waves, if the waves are bounded.)
Final calendar year weights are computed only for people who had an interview covering the control date5 and for whom data are obtained (either reported or imputed) for every month of the calendar year for which they were in scope for the survey. Other people are assigned final calendar year weights of zero. Some people who joined the household of an original sample person after the start of the panel are assigned nonzero calendar year weights for the second calendar year, if data are obtained for that period.
The full panel weighting scheme does not assign weights to people who enter the sample universe after Wave 1. Similarly, the calendar year weighting scheme does not assign weights to people who do not have an interview covering the control date. This group consists of (a) people who enter the sample universe after the first wave of interviewing for the calendar year and (b) people who were in the sample universe in the first wave of interviewing in the calendar year but did not have an interview covering the control date. For example, newborn infants and people leaving institutions who are entering the sample universe after Wave 1 are assigned full panel and calendar year 1 weights of zero. Note that the same people will receive positive calendar year 2 (CY2) weights if they are in the sample universe in the first wave of interviewing for CY2 and have an interview covering the control date for CY2.
The final panel and calendar year weights are constructed from the following three components:
Using Weights in the Topical Module Files
Using Weights in the Full Panel Files
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Page Last Modified: May 9, 2006
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