For 1970 through 1979, the questions used to elicit information on whether individuals were veterans and, if so, when they served, were the following: "Did [person] ever serve in the Armed Forces of the United States?" and, if yes, "When did he serve?" Beginning in 1980, the opening question changed to, "Did [person] ever serve on active duty in the Armed Forces of the United States?," while the second question remained the same.

The series of questions used in 1968-1969 was somewhat different. Interviewers asked, "Did [person] ever serve in the Armed Forces of the United States?" (If yes), "Was any of this service during a war?" "Was any of his service between June 27, 1950 and January 31, 1955?" "Was any of his service after January 31, 1955?" "Was any of his service after August 4, 1964?"

For example, in 1979 the list of possible responses was:

Thus, if a person served in both the Korean War and the Vietnam Era, the interviewer circled "VN." In general, recent wartime service was prioritized over earlier wartime service, and wartime service was prioritized over peacetime service. Beginning in 1990, however, this distinction breaks down since service during the Gulf War (from August 2, 1990 to February 28, 1991) was not distinguished from other post-Vietnam service. Therefore, a person who served during both the Vietnam Era and during the Gulf War would be classified as serving during the Vietnam Era, even though the Gulf War service was the more recent period of wartime service.

The Field Representative's Manual for 1968 provided the following clarification of what counted as "service in the Armed Forces":

The same definitions were repeated in the Manuals for subsequent years, with slight changes in wording but with no substantive changes until 1980.

In 1980, the Manual repeated these general points but added the following further clarification on how to classify responses to the opening question, "Did [person] ever serve on active duty in the Armed Forces of the United States?":

Beginning in 1982, the Manual repeated these points but specified "full-time" active duty and added as part of "active duty" service by "cadets appointed to one of the military academies, such as West Point, Naval Academy (Annapolis), etc." Beginning in 1984, the Manual added, "Also, do not include persons in the military service of a foreign nation."

The 1968, these values were: Korean War (June 27, 1950-January 31, 1955); Cold War (February 1, 1955-August 3, 1964); Vietnam (August 4, 1964 forward); Pre-1970 wartime, not Korea or Vietnam (namely, all persons reported to have served during a war and prior to June 27, 1950); and Peacetime only (namely, all persons reported to have served prior to June 27, 1950 but not during a war).

For 1970-1995, World War I (April 1917-November 1918) and World War II (September 1940-July 1947) are coded separately; for 1996, the two World Wars are combined. Veterans whose most recent periods of service occurred during the Korean War (June 1950-January 1955) can be consistently identified throughout the period from 1968 to 1996. The Vietnam Era, another consistently available code, is traditionally defined as the period between August, 1964 and April, 1975, but in the IPUMS NHIS data from 1968-1974, the length of the Vietnam era is necessarily truncated. Service during the post-Vietnam Era was identified beginning in 1976. The residual value "Other Service" must be interpreted relative to the specific values available for each year.

Thus, for example, all responses referring to World War service share a common first digit of "2." Additional detail available only in certain years is provided by the second digit, which, in this example, distinguishes between World War I (code 21), World War II (code 22), and World War I or II, unspecified (code 23). Changes in question wording (particularly the different phrasing used in 1968-1969) and the addition of more categories with the passage of time, allow only a limited number of categories, mentioned above, to be distinguished in all years, however.

For example, for 1968, wartime veterans are those who served during the Korean War, the Vietnam Era, or another period of wartime service (codes 30, 40, and 62), while peacetime veterans are those who served during the Cold War or during peacetime (codes 61 and 63). For 1976-1989, wartime veterans are, broadly speaking, those who served during World War I, World War II, the Korean War, or the Vietnam Era (codes 10 through 40), while peacetime veterans served during the post-Vietnam period (code 50) or during "other service" (code 70). For 1990 to 1996, however, wartime service cannot be fully distinguished from peacetime service. As noted above, service during the Gulf War in 1990 and 1991 was not separated from other post-Vietnam service on the survey form.