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Sex effects on mean delta wave amplitude and incidence in the (A) 9-year-old cohort and (B) the 12-year-old cohort. Within the 9-year-old cohort, neither delta-wave amplitude nor incidence differed between boys and girls. Within the 12-year-old cohort, boys had significantly (*) higher delta-wave amplitude than girls, but incidence was the same for both sexes. Data shown are for the first recording period.
