# Which counties have sample sizes large enough for accurate aggregation of CPS microdata?

**URL:** <https://forum.ipums.org/t/which-counties-have-sample-sizes-large-enough-for-accurate-aggregation-of-cps-microdata/2751>\
**Category:** CPS\
**Created:** [January 22, 2019, 9:02pm UTC](https://forum.ipums.org/t/which-counties-have-sample-sizes-large-enough-for-accurate-aggregation-of-cps-microdata/2751 "2019-01-22T21:02:00Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![jvandernaald](https://sea1.discourse-cdn.com/flex019/user_avatar/forum.ipums.org/jvandernaald/32/292_2.png) [@jvandernaald](https://forum.ipums.org/u/jvandernaald)\
**Post date:** [January 22, 2019, 9:02pm UTC](https://forum.ipums.org/t/which-counties-have-sample-sizes-large-enough-for-accurate-aggregation-of-cps-microdata/2751/1 "2019-01-22T21:02:00Z")

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I am working on a research project where I need to determine county-level union density in a single year. Because the sample size for particular counties in a single year are so low, I am pooling three years’ worth of data on union membership to increase the number of observations. Nevertheless, I know that certain counties have too small of a sample size to have accurate estimates at the county level.

Does IPUMS or CPS provide a list of counties whose microdata are unreliable for aggregating to the county level because the sampling is incomprehensive or too small?

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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 2, 2020, 9:14pm UTC](https://forum.ipums.org/t/which-counties-have-sample-sizes-large-enough-for-accurate-aggregation-of-cps-microdata/2751/2 "2020-07-02T21:14:52Z")

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Our apologies, this post seems to have slipped past our radar. Though you have likely found an answer to your question by now, we want to provide a response for you and for future users who may have a similar question. Due to the fact that the CPS is stratified at the state level (as outlined in the [Census Technical Paper 66](https://www.census.gov/prod/2006pubs/tp-66.pdf) on sample design and methodology), it is not designed to generate sub-state level estimates, making county-level estimates less reliable (as you indicated). However, according to the [March 2018 ASEC codebook](https://cps.ipums.org/cps/resources/codebooks/cpsmar18.pdf) (page 2-8), smaller metropolitan areas should not be used though large metropolitan areas such as NYC or LA are considered acceptable. The documentation provides general guidelines, but does not include a detailed list of counties. The decision about whether a county is reliable may be determined by the analyst according to their specific use case or if they pool enough adjacent months or years of data together.