The lower age limit for CLASSWKR differs for IPUMS-USA and IPUMS-CPS in some years. The 1980, 1990, and 2000 censuses and the ACS collected employment information on persons 16 and older, while earlier enumerations used an age limit of 14 and older. The CPS used an age limit of 15 for CLASSWKR beginning in 1988 and age 14 in earlier years. Other differences between IPUMS-CPS and IPUMS-USA relate to the degree of detail in the CLASSWKR variable. The distinction between incorporated and unincorporated enterprises for self-employed persons was first made in 1970 in the census and not until 1988 in CPS. The CPS does not distinguish between wage and salary workers in private industry and those at non-profit organizations, as the 1990 and 2000 censuses and the ACS do; the two groups must be combined to achieve comparability between IPUMS-USA and IPUMS-CPS. Members of the armed forces are subsumed into the general category of federal government employees in the census, beginning in 1960, and in the ACS. The distinction between federal, state, and local government employees was initiated in the census in 1970 and in the CPS in 1988. Analysts who wish to exploit data from the full time series and combine data from IPUMS-USA and IPUMS-CPS can achieve comparability by focusing on four main classifications: self-employed persons, private wage and salary workers (including persons working for non-profit organizations), government workers (including members of the armed forces), and persons working without pay in family businesses.