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Table 2 Summary statistics for the IBI analysis

From: Impact of energy availability and physical activity on variation in fertility across human populations

Income Group

Sample Size (N)

Mean IBI (months)

SE (months)

Modal IBI (months)

Middle-income

29,016

30.5

18.5

24

Low-income

33,949

27.0

13.0

26

Natural fertility

276

34.9

19.7

24

US WIC nonparticipant

438,441

39.8

24.3

21

US WIC participant

331,128

41.5

26.8

21

  1. Income groups classified by country GNI per capita: low-income =  ≤ US $1085; middle-income = US $1086–13,205; high-income =  ≥ US $13,205