* For each survey year, poverty status is based on the ratio of the family's income to federal poverty levels, as calculated by the U.S. Census Bureau. In addition to family income, federal poverty levels take into account family size and the number of children in the family. A larger ratio of income relative to poverty level indicates increasing family income and thus decreasing poverty status. Because of high levels of missing income data in NHIS, poverty status was determined from the 2001--2007 NHIS multiple imputed income and earnings data files.
† Estimates are based on household interviews of a sample of the civilian, noninstitutionalized U.S. population. One child aged <18 years was randomly selected per family; a knowledgeable adult provided information for each child. Prevalence of impairments or health problems resulting in activity limitations is based on a question that asked, "Does [the sample child] have an impairment or health problem that limits [his/her] ability to crawl, walk, run, or play?" Unknowns with respect to impairments are excluded from the denominators.
§ 95% confidence interval.