Thank you for your reply! I looked at the appendix you attached but this did not answer my problems. It is true from the attachment I can figure out which counties go in which SEA. The problem is the SEAcode are also something different from the SEAs. It mostly coincides with the area, but for instance, Pennsylvania SEA 1 corresponds to both SEA code 360 and 363 (https://usa.ipums.org/usa/resources/c… : p.203). This is problematic for me when I only have the information on which counties are in which SEA in 1960, but the data I want to crosswalk is at the SEA level. If for example I have population of SEA 1 in 1960, I cannot figure out whether this corresponds to SEA code 360 or 363. And the IPUMS census 1940 also uses the codes, and not the SEA itself. Do you know of any documentation that specifies under which SEA code the corresponding SEA or counties are? I would really appreciate any guidance in this! Thank you!