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Table 2 Relative frequencies of Stomach ache among grade 5, 7 and 9 students, distributed by gender and menarche

From: Explaining differential item functioning focusing on the crucial role of external information – an example from the measurement of adolescent mental health

  Boys Girls no period Girls period
n = 9153 n = 3676 n = 5446
Every day 2.1% 3.7% 5.5%
More than once a week 5.5% 8.0% 13.6%
Once a week 11.3% 15.5% 16.8%
About once a month 29.0% 28.7% 44.0%
Seldom or never 52.2% 44.1% 20.0%