TABLE 2

Sample Demographics

Total Sample6 mo12 mo24 mo36 mo
Sample size, n359342335280258
Age, mean ± SD, moNA6.4 ± 0.512.9 ± 1.225.1 ± 1.037.3 ± 1.5
Gender, % girls5555555454
Race, % white
% African American646566**70***71***
3130282424
Parity (median [25th, 75th percentile])1 (0, 1)1 (0, 1)1 (0, 1)1 (0, 1)1 (0, 1)
Maternal education at birth
 (median years [25th, 75th percentile])16
(14, 16)16
(14, 16)*16
(14, 16)***16
(14, 16)***16
(14, 16)**
Family income at birth
 (median in $1000s [25th, 75th percentile])50 000–60 000
(25–30, 80–90)50 000–60 000
(25–30, 80–90)50 000–60 000
(25–30, 80–90)**60 000–70 000
(30–40, 80–90)***60 000–70 000
(30–40, 80–90)**
  • Demographic differences between retained and missing subjects via χ2 or Mann-Whitney U test, *P < .05, **P < .01; ***P < .001. NA, not available.