Response rates and selection problems, with emphasis on mental health variables and DNA sampling, in large population-based, cross-sectional and longitudinal studies of adolescents in Norway

Table 2 Number of participants in the longitudinal studies¹ in Oslo and Hedmark according to gender and ethnicity

		Baseline 2001	Follow-up 2004
		Participants	Participated and accepted linkage of data²		Participants who accepted linkage of data²and provide DNA³
		N	n	%	n	%
Oslo
Total⁴		3811	2489	65.3	2433	97.7
Ethnicity⁶:
Norway/western	Girls	1424	1103	77.5	1078	97.7
	Boys	1456	906	62.2	893	98.6
Non-western	Girls	422	261	61.8	249	95.4
	Boys	425	192	45.2	186	96.9
Hedmark
Total⁵		1939	827	42.7	796	96.3
Ethnicity⁷:
Norway/western	Girls	950	494	52.0	477	96.6
	Boys	931	317	34.0	303	95.6
Non-western	Girls	17	6	35.3	6	100.0
	Boys	25	9	36.0	9	100.0

¹Participated both in the baseline study 2001 and in the follow-up 2004;
²Participants accepting linkage of data to other health surveys and registers, i.e. linkage of data to the baseline data;
³ Percent providing DNA among participants accepting linkage of data; ^4,5Due to missing data in Oslo on gender and/or ethnicity (84), and in Hedmark on ethnicity (16), the numbers will not sum up to the total numbers in Oslo, nor Hedmark; ^6,7Based on self-reported native country of mother and father.

ISSN: 1471-2458