Certain populations, e.g. members of the Armed Forces, sere counted using special rules. American citizens abroad for an extended period of time were not included except for those employees of the U.S. government or members of the U.S. Armed Forces. They were counted as being in their home of record for purposes of apportionment. For each person in a household, the sex of that person is asked. There has been no change in this over the course of the census periods covered here.
Race is always self-reported and has nothing to do with scientific definitions of biological stock.
In 1970 persons of racial mixtures were classified according to the race of the Father if he was in the household and his race could be determined. Persons having Spanish surname identified by matching with a list of 8,000 such names. In the remaining states, persons of Spanish language. In 1990 the Other classes were asked to enter the country of identity, e.g. Argentinean, Colombian, etc. Thus there are significant differences between 1970 and 1980 and minor differences for 1990.