I downloaded a data extract from IPUMS and want to compare those data with a different data source. The common (ideally) merge variable would be State-specific county. However, the county schema in IPUMS is not a unique level. How should I best go about this?
While values for COUNTY are not unique across states, they are unique within states. Thus, you can combine COUNTY with STATE information (STATEFIP) to distinguish among counties in different states. You could then merge the IPUMS file to a different county-level file using state and county as the unique identifiers.
I hope this helps. Let me know if I misunderstood your question, or if there is something about your non-IPUMS data source that would prevent such a merge.