Table 1.

Study population characteristics.

Characteristics	All (n = 419)		Boys (n = 191)		Girls (n = 228)
	n	%	n	%	n	%
Sociodemographic variables
Age (years)
[13.5–14.5] years	113	27	48	25.1	65	28.5
[14.5–15.5] years	273	65.2	127	66.5	146	64
>15.5 years	33	7.9	16	8.4	17	7.5
Country of birth
Belgium	337	80.4	158	82.7	179	78.5
EU	35	8.4	15	7.9	20	8.8
Outside EU	46	11	18	9.4	28	12.3
Missing	1	0.2	0	0	1	0.4
Household educational attainment
Primary	24	5.7	6	3.1	18	7.9
Secondary	139	33.2	53	27.7	86	37.7
Tertiary	249	59.4	131	68.6	118	51.8
Missing	7	1.7	1	0.5	6	2.6
Equivalent income (monthly)
0–1250 Euro	89	21.2	40	20.9	49	21.5
1251–1600 Euro	73	17.4	35	18.3	38	16.7
1601–2000 Euro	71	16.9	31	16.2	40	17.5
>2000 Euro	118	28.2	57	29.8	61	26.8
Missing	68	16.2	28	14.7	40	17.5
Perceived income adequacy
Difficult	119	28.4	56	29.3	63	27.6
Rather easy	133	31.7	62	32.5	71	31.1
Easy to very easy	160	38.2	70	36.6	90	39.5
Missing	7	1.7	3	1.6	4	1.8
Area deprivation index
0–5.3%	103	24.6	42	22	61	26.8
5.4–9.3%	105	25.1	42	22	63	27.6
9.4–15.5%	103	24.6	48	25.1	55	24.1
>15.5%	107	25.5	58	30.4	49	25.1
Missing	1	0.2	1	0.5	0	0
Urbanicity (Eurostat)
Rural areas	59	14.1	17	8.9	42	18.4
Towns/suburbs	303	72.3	154	80.6	149	65.4
Cities	57	13.6	20	10.5	37	16.2
Population density
<600 inhabitants/km2	268	64	121	63.4	147	64.5
≥600 inhabitants/km2	151	36	70	36.6	81	35.5
Lifestyle variables
Smoking
Never or once	400	95.5	180	94.2	220	96.5
Less than daily	8	1.9	5	2.6	3	1.3
Daily	10	2.4	5	2.6	5	2.2
Missing	1	0.2	1	0.5	0	0
Alcohol use
Never	263	62.8	128	67	135	59.2
<1 consumption monthly	93	22.2	37	19.4	56	24.6
Monthly or more	61	14.6	25	13.1	36	15.8
Missing	2	0.5	1	0.5	1	0.4
Active sportsa
Never or <1 x/week	63	15	26	13.6	37	16.2
1–2 times a week	152	36.3	59	30.9	93	40.8
>2 times a week	202	48.2	104	54.5	98	43
Missing	2	0.5	2	1	0	0
Physical health characteristics
Body mass indexb
Underweight	34	8.1	16	8.4	18	7.9
Normal weight	302	72.1	147	77	155	68
Overweight, obese	83	19.8	28	14.7	55	24.1
Atopic diseasec
Yes	129	30.8	66	34.6	63	27.6
No	281	67.1	121	63.4	160	70.2
Missing	9	2.1	4	2.1	5	2.2
Infections (past year)
Yes	244	58.2	110	57.6	134	58.8
No	162	38.7	76	39.8	86	37.7
Missing	13	3.1	5	2.6	8	3.5
Mental health characteristics
Perceived stress (past 3 months)
Low	113	27	73	38.2	40	17.5
Medium	164	39.1	76	39.8	88	38.6
High	139	33.2	39	20.4	100	43.9
Missing	3	0.7	3	1.6	0	0
Optimal vitalityd
Yes	56	13.4	27	14.1	29	12.7
No	360	85.9	161	84.3	199	87.3
Missing	3	0.7	3	1.6	0	0
Perceived happiness
Low	34	8.1	7	3.7	27	11.8
High	382	91.2	181	94.8	201	88.2
Missing	3	0.7	3	1.6	0	0

Data in numbers (n) and percentages (%), ^aphysical activity that causes sweating and/or heavy breathing, ^bBody Mass Index classes based on age- and sex-specific Belgian growth curves, ^cdoctor-diagnosed asthma and/or atopic dermatitis and/or allergic rhinitis, ^dvitality score > 77.4 (study population mean plus one standard deviation). EU, European Union; SDQ, Strengths and Difficulties Questionnaire.