For 1969-1996, for persons who worked during the preceding two weeks or who had a job but were not at work during those two weeks, information was collected on the current job; for persons who were currently unemployed (looking for work or on lay-off) or who were waiting for a new job to begin, the question referred to the most recent full-time (35+ hours per week) civilian (non-military) job held for at least two weeks. For 1997-2003, the question applied only to the current job of sample adults who worked during the preceding week. For 2004 forward, for sample adults who had worked during the preceding week or who had a job but were not at work during that week, information was collected on the current job. Also for 2004 forward, for sample adults who were not working or temporarily absent from a job during the previous week but who had ever worked at a job or business, information was collected on the most recent job (for persons under age 65) or on the longest-held job (for persons age 65 or older).

Prior to 1997, interviewers generally inferred class of worker from information provided in response to earlier questions, namely: "Who does [person] work for?"; "What kind of business or industry is this?"; and "What kind of work is [person] doing?" As the Field Representative's Manual for 1969 and later years stated, "The information given in answer to questions 41a-c will usually be sufficient for identifying 'class of worker.' If the information previously supplied is not adequate for this purpose, ask additional questions as necessary, for example, 'Did he work for himself in his own business, or did he work for someone else?'" Beginning in 1997, the information was systematically provided by respondents, who selected the appropriate category from choices listed on a card.

The first digit groups together broad categories available across years; the second digit provides detail available only in some years. Thus, for example, all government employee classifications share a first digit of "3," while the second digit distinguishes between federal, state, and local government employees (because the latter two groups were separately identified only for 1974 forward). Similarly, all self-employed persons receive a first-digit code of "4," while the second digit distinguishes between self-employment in incorporated and non-incorporated businesses (information only available for 1974-1996).