# What is "O" as a CountyFIP? + Why is largest county missing?

**URL:** <https://forum.ipums.org/t/what-is-o-as-a-countyfip-why-is-largest-county-missing/4221>\
**Category:** General\
**Created:** [July 7, 2021, 4:33pm UTC](https://forum.ipums.org/t/what-is-o-as-a-countyfip-why-is-largest-county-missing/4221 "2021-07-07T16:33:24Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Martha\_Lagace](https://avatars.discourse-cdn.com/v4/letter/m/4da419/32.png) [@Martha\_Lagace](https://forum.ipums.org/u/Martha_Lagace)\
**Post date:** [July 7, 2021, 4:33pm UTC](https://forum.ipums.org/t/what-is-o-as-a-countyfip-why-is-largest-county-missing/4221/1 "2021-07-07T16:33:24Z")

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Question: The most populous county in Georgia is Fulton County (CountyFIP 121). However, using “Analyze Data Online” in IPUMS USA, and entering CountyFIP as the column variable, BPLD as the row variable, and the StateFIP for Georgia (13) as the Filter, I received 2 curious results: a column with “0,” and none for Fulton County. And there are far fewer counties in my columns than Georgia’s 159 counties - just a few dozen. What am I doing wrong? Does “0” just equal “everything else”? Why isn’t Fulton County there?

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**Author:** ![Grace\_Cooper](https://sea1.discourse-cdn.com/flex019/user_avatar/forum.ipums.org/grace_cooper/32/376_2.png) [@Grace\_Cooper](https://forum.ipums.org/u/Grace_Cooper)\
**Post date:** [July 9, 2021, 10:04pm UTC](https://forum.ipums.org/t/what-is-o-as-a-countyfip-why-is-largest-county-missing/4221/2 "2021-07-09T22:04:35Z")

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About half of US counties cannot be identified in the ACS public use microdata sample data. IPUMS identifies counties, where possible, from other low-level geographic identifiers; the lowest geographic identifier available in the public use data for 1950-forward is a [public use microdata area (PUMA)](https://usa.ipums.org/usa-action/variables/puma#description_section). I am linking a [blogpost](http://blog.popdata.org/ipums-faqs-missing-u-s-counties/) on the topic; the [COUNTYFIP description page](https://usa.ipums.org/usa-action/variables/countyfip#description_section) includes a spreadsheet of identified counties in the 1950-forward data as well. Fulton county is not identified in the 2012-forward ACS data; it can be surprising when a populous county is not identified, but occasionally the way PUMA boundaries align does not permit for identification of even very populous places.

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**Author:** ![Martha\_Lagace](https://avatars.discourse-cdn.com/v4/letter/m/4da419/32.png) [@Martha\_Lagace](https://forum.ipums.org/u/Martha_Lagace)\
**Post date:** [July 9, 2021, 10:23pm UTC](https://forum.ipums.org/t/what-is-o-as-a-countyfip-why-is-largest-county-missing/4221/3 "2021-07-09T22:23:59Z")

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Thank you very much.