Table 2.

Prevalence rates of mental health problems by relative age at school.

Age-group (years), grade	N	CBCL Total Problems scores (mean, standard error)			P-value
		Old	Middle	Young
5–6 (grade 2)	9,069	17.10 (0.27)	17.81 (0.27)	18.02 (0.28)	0.017 *
Cross-section	1,332	15.64 (0.64)	16.03 (0.67)	16.27 (0.65)	0.588 *
RCT	7,737	17.35 (0.30)	18.10 (0.30)	18.32 (0.30)	0.018 *
7–12 (grades 4–8)	1,602	18.73 (0.67)	18.31 (0.68)	18.77 (0.62)	0.696 *
13–15 (i.e. grades 1–3 of Dutch secondary school)	1,237	16.88 (0.86)	17.11 (0.71)	17.57 (0.80)	0.489 *
		Prevalence rates of CHP assessed psychosocial problems ***
		Old	Middle	Young
5–6 (grade 2)	9,172	297/2,990 (9.9%)	367/3,064 (12.0%)	422/3,118 (13.5%)	<0.0001 (2x) **
Cross-section	1,435	54/485 (11.1%)	660/464 (12.9%)	74/486 (15.2%)	0.166/0.059**
RCT	7,737	243/2,507 (9.7%)	308/2,601 (11.8%)	347/2,633 (13.2%)	<0.0001 (2x) **
7–12 (grades 4–8)	1,701	59/562 (10.5%)	61/535 (11.4%)	67/604 (11.1%)	0.888/0.751**
13–15 (i.e. grades 1–3 of Dutch secondary school)	1,344	52/415 (12.5%)	53/478 (11.1%)	50/451 (11.1%)	0.746/0.513 **

Ages 5–6 concerns the combined cross-sectional and RCT data, the other two ages only concern the cross-sectional data. Data on the CBCL were lacking for 309 children in the cross-sectional study (these did not differ from the remaining regarding any of the variables indicated in Table 1). CBCL = Child Behavior Checklist. CHP = Child Health Professional (doctor or nurse).

* Of Spearman correlation coefficient with relative age.

** Chi-square test and Mantel's test for linearity, respectively, for differences in prevalence rates by relative age.

*** Number of children having a problem/total number of children.