I’m attempting to identify which counties in the US are the most representative of the country as a whole before I continue on with my research using the 5% 2019 ACS. I realize that ACS microdata do not always go down to the county-level. If I’m reading the documentation correctly, it seems that PUMAs are the closest geographic equivalent: they sometimes consist of a fraction of a county, an entire county, or multiple counties depending on populations. I’m curious if there are distinct issues with using the State and County Fips code variables together to try to generate a county indicator. Are there any specific types of counties that missing from ACS micro data, or the omitted counties effectively random? In other words - should I be concerned in comparing the counties I can identify to the population averages generated from a particular mico data file?