Table 1. Descriptive statistics.

Comparison of frequencies in the DOGSS cohort and the attrition cohort.

		DOGSS cohort (N = 450)	Attrition cohort (N = 410)	P A
Sex	Girls	41.8%	31.7%	0.00
Screen-positives	Cases (NDP+non-NDP)	54.9%	58.5%	0.28
ADHD	ADHD screen-positive	21.1%	30.0%	0.00
Screen-negatives	Screen-negative cotwin	34.9%	32.7%	0.38
	Screen-negative control	10.2%	8.8%	0.47
Parental education level B	Both parents have primary school or less (9 years of school)	3.1%	0.5%	0.01
	At least one of the parents has secondary school (10–12 years)	35.6%	41.2%	0.04
	At least one parent with university studies or equivalent	39.1%	35.1%	0.24

Note: Missing information on 22.2% of the DOGSS cohort, and 23.2% of the attrition cohort.

^A P value for comparison of distribution of qualitative variables between the DOGSS cohort and the attrition cohort with a Chi-square test

^B Parental education level: A combination of both parents’ education levels.