Total combined family income included the respondent's own income plus the income of all co-resident family members, including cohabiting partners and related armed forces members living at home. It did not include the value of noncash benefits such as Food Stamps, Medicaid, Medicare, and public housing.

If the reported family income figure was the same or higher than the poverty threshold for families of that size and age composition, the individual (and all members of the family) was considered "above poverty" and received a code of 1 in POORYN. If the reported family income figure was less than the Census Bureau's poverty cut-off for families of that size and age composition, the individual (and all other members of the family) was classified as "poor" and received a code of 2 in POORYN.

Poverty data in IPUMS NHIS and in U.S. government statistics generally are based on a definition established by the Social Security Administration in 1964 and subsequently modified by Federal interagency committees in 1969 and 1980. The Office of Management and Budget's (OMB) Directive 14 prescribes this definition as the official poverty measure for federal agencies to use in their statistical work.

At the core of this definition was the 1961 economy food plan, the least costly of four nutritionally adequate food plans designed by the Department of Agriculture. It was determined from the Agriculture Department's 1955 survey of food consumption that families of three or more persons spend approximately one-third of their income on food; hence, the poverty level for these families was set at three times the cost of the economy food plan. For smaller families and for persons living alone, the cost of the economy food plan was multiplied by factors that were slightly higher, to compensate for the relatively larger fixed expenses for these smaller households. The federally established poverty thresholds are revised annually to allow for changes in the cost of living, as reflected in the Consumer Price Index. The poverty thresholds are the same for all parts of the country; they are not adjusted for regional, state, or local variations in the cost of living.

For families where the number of children under age 18 in the family is equivalent to the number of family members, family income is presumed to be nonexistent, and the ratio of such nonexistent income to the poverty line is undefinable. The documentation for the public use files of NHIS does not specify how such cases were treated prior to 1997, but it is logical to assume that they were simply grouped with other "unknown" cases. The value label noting that POORYN's "Unknown" category includes "Undefinable" for 1997-forward does not, therefore, highlight any serious comparability problem.