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Table 3 Descriptive characteristics for variables used in multilevel models (%)

From: Urban–rural disparity in the relationship between ambient air pollution and preterm birth

Variable

Hubei

East

Middle

West

Total

429,865

228,603

138,310

62,952

GA (weeks)

 Mean (Std)

39.24 (1.27)

39.32 (1.26)

39.14 (1.25)

39.17 (1.32)

 < 37

2.98

2.91

2.94

3.32

Residence types

 City propera

31.98

40.04

23.04

22.36

 County town

10.45

9.94

9.10

15.24

 Suburb

9.10

13.68

4.90

1.70

 General town

41.19

31.81

58.65

36.89

 Countryside

7.28

4.53

4.31

23.81

PM2.5 exposure (30 μg/m3)

 Mean (Std)

3.10 (1.18)

3.63 (1.21)

2.87 (0.61)

1.69 (0.53)

Nationality

 Han (Main)a

96.89

99.63

99.35

81.55

 Minority

3.11

0.37

0.65

18.45

Age (years)

 Mean (Std)

26.83 (4.50)

26.95 (4.49)

26.60 (4.32)

26.91 (4.93)

 < 20

2.29

1.95

2.08

3.96

 20–24

30.49

29.85

32.04

29.39

 25–29a

43.98

44.22

44.72

41.50

 30–34

16.49

17.13

15.38

16.65

 ≥ 35

6.75

6.85

5.79

8.49

Educational level

 Elementary

19.30

21.21

15.44

20.85

 Secondarya

74.79

70.70

81.20

75.55

 Tertiary

5.91

8.09

3.36

3.59

Frequency of health checks

 < 5a

78.31

63.25

95.19

95.91

 ≥ 5

21.69

36.75

4.81

4.09

Parity

 Firstborn

75.41

73.38

81.01

70.45

 Non-Firstborna

24.59

26.62

18.99

29.55

Infant sex

 Malea

54.07

54.72

53.82

52.28

 Female

45.93

45.28

46.18

47.72

Income (1,000 CNY)

 Mean (Std)

14.32 (7.20)

15.89 (8.15)

13.69 (4.82)

9.98 (5.75)

  1. Std, standard deviation; CNY, Chinese yuan
  2. a” indicates reference group