Sample Probability Weights

How are the survey probability weights (variable PERWT) constructed? What factors are considered when creating the weights?

The answer depends on the sample you are analyzing. There is a lot of detail documented on weights in IPUMS USA. A good place to begin reviewing this information is the description and comparability tabs for PERWT as well as the sample weights section of the Intro to IPUMS USA Guide. The person-level weights generally represent the inverse probability of selection into the sample. In flat decennial census samples, including the full count files, the weights are identical for each person since each U.S. resident had the same probability of being included in the data. In a full count file, each individual gets assigned a PERWT of one whereas in a flat subsample, such as the 2010 10% file, each individual gets a PERWT of ten. This is done so that the sum of PERWT equals the total US resident population in that year. More information about the construction of PERWT in weighted historical samples can be found in the IPUMS User Guide on Sample Designs.

In the American Community Survey (ACS), the process of weight creation is documented in Chapter 11 of the Design and Methodology Report. This processing is done by the Census Bureau and the weights are released as variables that IPUMS integrates into our extract system. The report notes that weights are used to bring the characteristics of the sample more into agreement with those of the full population by compensating for differences in sampling rates across areas, differences between the full sample and the interviewed sample, and differences between the sample and independent estimates of basic demographic characteristics. In short, the ACS uses county-level estimates of the number of housing units and persons by age, sex, race, and Hispanic origin produced by the Population Estimates Program (PEP) to construct weights such that the weighted estimates for each group conform to the PEP values (see Population Controls in the ACS User Handbook). Additional steps including the monthly sample factor and non-interview adjustment are documented in the linked report.

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