In the 1981 Field Representative's Manual, "regular milk" was defined as any kind of milk other than mother's or wet nurse's milk, such as cow's or goat's milk or powdered milk. Formula was defined as "a mixture or milk substitute which is fed to babies." A definition for the term "regular basis" was not given, and therefore was assumed to be respondent defined.

Interviewers recorded the age of the child when he/she was first fed formula or regular milk on a daily basis in units of days, weeks or months. IPUMS NHIS recodes this variable in months.