BMC Public Health 2004, 4 http://www.biomedcentral.com/1471-2458/4/16 Table 4: Comparison of household characteristics between the NHIS (1994/1995) and the U.S. Census of households (2000). NHIS Survey 1994/1995 U.S. Census of Households 2000* 104,706,000 9,656,000 (9.2) 15,142,000 (14.5) 14,128,000 (13.5) 12,398,000 (11.8) 8,281,000 (7.9) 44,505,000 (42.5) 595,000 (0.6) 34,666,000 (33.1) 17,152,000 (16.4) 15,309,000 (14.6) 6,981,000 (6.7) 2,445,000 (2.3) 1,428,000 (1.4) 101,281,000 (96.7) 3,425,000 (3.3) 87,767,000 (83.8) 16,939,000 (16.2) 86,203,000 (82.3) 18,502,000 (17.7) 87,213,000 (83.3) 17,493,000 (16.7) 66,676,000 (63.7) 38,029,000 (36.3) 16,760,000 (16.0) 87,946,000 (84.0) 80,429,000 (76.8) 24,276,000 (23.2) Page 5 of 7 (page number not for citation purposes) Number of households with completed survey Income <$10,000 $10,000 to <$20,000 $20,000 to <$30,000 $30,000 to <$40,000 $40,000 to <$50,000 $50,000 + Unknown/Missing Households Size Two members Three members Four members Five members Six members Seven or more members Age of household members Without members under 1 With members under 1 Without members under 6 With members under 6 Without members 6 – 11 With members under 6 – 11 Without members 12 – 17 With members 12 – 17 Without members under 18 With members under 18 Without members 18 – 64 With members 18 – 64 Without members 65 + With members 65 + 27,978 3,403 (12.2) 4,788 (17.1) 4,042 (14.5) 3,238 (11.6) 2,452 (8.8) 5,174 (18.5) 4,881 (17.5) 8,811 (31.5) 4,634 (16.6) 4,063 (14.5) 1,979 (7.1) 722 (2.6) 565 (2.0) 27,480 (98.2) 498 (1.8) 25,463 (91.0) 2,515 (9.0) 26,388 (94.3) 1,590 (5.7) 26,604 (95.1) 1,374 (4.9) 22,499 (80.42) 5,479 (19.6) 10,959 (39.2) 17,019 (60.8) 22,498 (80.4) 5,480 (19.6) *Source: Fields JM, Casper LM. America's Families and Living Arrangements: March 2000. Current Population Reports, P20-537, U.S. Census Bureau, 2001. Table 2 presents individual and household characteristics of the four groups. No statistically significant differences were found; however, there was a higher percentage of females (68.7%, odds ratio [OR]: 1.8, 95% confidence interval [CI]: 0.7–5.1), of African-Americans (25.0%, OR: 2.8, 95% CI: 0.9–9.0), and of households with lower socio-economic status (<$20,000; 37.5%, OR:1.8, 95% CI: 0.5–6.9) in the multiple familial group. Also, none of the 8 households in this group was located in western states. Table 3 details the number of households with at least one resident with a birth defect and shows projections for the total number of these households in the United States and by region using the 2000 U.S. Census report. Western states have the highest rate of households including resi- dents with birth defects in two of the four groups, single non-familial and single familial. Of the 54 households with single familial cases, 39 had two and 5 had three family members with the same con- dition. One household included a pair of male twins with 47, XYY karyotype. In the remaining 10 households, the affected family members had different types of birth defects (Table 5). Discussion The objective of this study was to determine whether the National Health Interview Survey is useful in identifying households with families who are informative in the genetic study of birth defects. We used the Survey to esti- mate the total number of households where these families live by geographical region. It was thought that this infor- mation could be used by genetic researchers to target areas where the likelihood of finding these families is greater and to anticipate the needed resources with a better understanding of the scope of the task. Unfortunately, the