Frequently Asked Questions

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I'm using the American Community Survey (ACS) Public Use Microdata Sample (PUMS) files. Why don't my estimates match what's published on American FactFinder (AFF)?

Public Use Microdata Sample (PUMS) files include a subset of cases from the full confidential ACS microdata file. Typically, the PUMS files include only about two-thirds of the cases contained in the larger confidential dataset that is used to make tables for AFF. The smaller number of cases in the PUMS means that PUMS estimates will be different and slightly less reliable than estimates in AFF.

The Census Bureau makes additional modifications to the PUMS files to protect the confidentiality of respondents. These modifications can include, for example, the "top-coding" of continuous variables with outlying values (such as income or transportation time) and the reduction of category availability for variables with hundreds of categories (such as ancestry or birthplace). These changes can also cause PUMS estimates to differ slightly from those in AFF.

To confirm that you are applying weights correctly, please refer to the user verification files on the ACS website.


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