Why do some counties appear to be completely missing in CPS data sets?

For example, for the variable “COUNTY,” Cook, IL (Chicago) and Harris, TX (Houston) seem to have zero records in the CPS over the last decade (I am specifically looking at ~2004-2015) despite being 2 of the most populous US counties. When looking at the county codes page (https://cps.ipums.org/cps/codes/count…) they are missing from the list. What is going on here?

Identification of CPS households by COUNTY is limited by confidentiality restrictions on public use data and relativley small counties across the US. As a result, not all counties have the necessary number of households required to preserve confidentiality. Therefore, only about 45% of all households are located in an identifiable county