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Table 2 Descriptive statistics for birth weight, small for gestational age, and preterm births by quartiles of traffic variables, land use, and median household income among all singleton births born in Eastern Massachusetts between 1996 and 2002

From: The effects of socioeconomic status and indices of physical environment on reduced birth weight and preterm births in Eastern Massachusetts

Continuous variables (quartiles) Mean birth weight (SD) Small for gestational age N (%) Preterm birth N (%) Comparison group (≥ 37 weeks; ≥ 2500 grams) N (%)
Cumulative traffic density/1000 (vehicles-km)         
   ≤ 261.0 3469 (539) 9441 22.4 7187 21.7 89628 25.6
   > 261.0 – 704.6 3427 (526) 10398 24.7 8105 24.4 87756 25.0
   > 704.6 – 1602.0 3398 (564) 10944 26.0 8628 26.0 86674 24.7
   > 1602.0 3368 (569) 11256 26.7 9207 27.7 85785 24.5
   Missing (688) 3413 (560) 70 0.2 57 0.2 561 0.2
Distance to primary highways (meters)         
   ≤ 1107 3398 (555) 11067 26.3 8460 25.5 86888 24.8
   > 1107 – 2150 3403 (565) 10741 25.5 8496 25.6 87196 24.9
   > 2150 – 3745 3413 (563) 10482 24.9 8422 25.4 87529 25.0
   > 3750 3448 (554) 9819 23.3 7806 23.5 88791 25.3
% land use for recreation and conservation (open space)         
   ≤ 4.0 3363 (568) 11184 26.6 8905 26.8 81668 23.3
   > 4.0 – 9.4 3403 (567) 10861 25.8 8711 26.3 86966 24.8
   > 9.4 – 17.8 3447 (547) 9658 22.9 7182 21.6 85787 24.5
   > 17.8 3458 (546) 9210 21.9 7328 22.1 87770 25.0
   Missing (10470) 3319 (597) 1196 2.8 1058 3.2 8213 2.3
Median household income (in US $)         
   ≤ 46336 3303 (581) 12853 30.5 10736 32.4 83081 23.7
   > 46336 – 62773 3406 (564) 10984 26.1 8405 25.3 86917 24.8
   > 62773 – 78583 3467 (547) 9778 23.2 7253 21.9 89304 25.5
   > 78583 3486 (526) 8494 20.2 6790 20.5 91102 26.0
  1. N – number; SD – standard deviation