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 1 Number of invitees and number of participants in the cross-sectional studies in 2001 and 2004, and three-year follow-up of samples from 2001

	Invitees	Participated		n and % of participants accepting linkage of data to other health surveys and registers		n and % of participants accepting DNA (Cyto-brush)
Study	N	n	%	n	%	n	%
Cross-sectional data collections
15/16 years old, 2001
Oslo, school based (sample 1) ¹	4273	3811	89.2	3433	90.1
Hedmark, school-based (sample 2) ¹	2197	1939	88.3	1791	92.4
TOTAL, Oslo and Hedmark	6470	5750	88.9	5224	90.9
18/19 year old, 2004
Oslo, school-based (sample 3) ¹	3659	3308	90.4	3036	91.8	3095	93.6
Three years follow-up of samples from 2001
18/19 years old, 2004
Oslo total (sample 4) ¹	3550	2735	77.0	2489	91.0	2527	92.4
School-based	2466²	2269	92.0	2105	92.8	2145	94.5
Postal-based	1084³	466	43.0	384	82.4	382	82.0
Hedmark, postal-based (sample 5) ¹	1683⁴	933	55.4	827	88.6	828	88.7
TOTAL, Oslo and Hedmark	5233	3668	70.1	3316	90.4	3355	91.5

Number of participants who agreed to link their data to other health studies and registries and number who agreed to DNA analyses.
¹Refers to samples 1-5 in the float chart (Figure 1);
²1193 individuals did not take part in the baseline study in 2001 (935 were new individuals and 258 individuals took part in a similar study in 2000). This explains why there are 1193 less individuals invited to the follow-up of the Oslo, school-based, as compared with the cross-sectional, school-based;
³173 individuals out of 3811, who participated in the baseline survey in Oslo in 2001 and were not on school lists in 2004, did not consent to a new invitation, in addition 61 had unknown address, thus 1084 were invited by post to the follow-up in 2004;
⁴229 individuals out of 1939 who participated in the baseline survey in Hedmark in 2001 did not consent to a new invitation, in addition 27 had unknown address, thus 1683 were invited in 2004.

ISSN: 1471-2458