As evident in the question wording quoted in the description of the variable, diagnosis "by a doctor" was required in 1989 and 1991, while diagnosis by "a doctor or other health professional" was acceptable for 1999 forward. In 1989, the survey not only excluded "other health professionals" from the question wording but also reinforced the point, in the instructions to interviewers, that only doctors' diagnoses were acceptable. The 1989 Field Representative's Manual specifically directed interviewers to probe persons who indicated a diagnosis by a health professional other than a doctor, to determine if this other health professional was relaying the information from a medical doctor, and only then mark to "Yes."

The 1976 question poses the most serious challenges to comparability. In 1976, persons could answer for other family or household members, while self-reporting was required in later years. Proxy reporting yields less complete and accurate responses than self-reporting, at least when minor medical problems and brief contacts with health care professionals are the subject. Researchers should thus exercise particular caution when comparing results between 1976 and all later years.