The lower age limit for WKSWORK2 differs for IPUMS-USA and IPUMS-CPS. The 1980, 1990, and 2000 censuses and the ACS collected employment information on persons 16 and older, while the CPS used an age limit of 14 and older for 1976 to 1979 and 15 and older beginning in 1980. Members of the armed forces were not asked this question in the survey until 1990. The census and ACS data have incorporated responses from armed forces personnel before 1990 and continue to do so. Until the 1980 census, census data on weeks worked during the previous year were available only in the form of intervals. Fortunately, the same intervals are used in IPUMS-USA and IPUMS-CPS, so researchers wishing to combine data from several decades and the two sources can use the compatible WKSWORK2 variable.