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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

From: 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

  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
  1. Number of participants who agreed to link their data to other health studies and registries and number who agreed to DNA analyses.
  2. 1Refers to samples 1-5 in the float chart (Figure 1);
  3. 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;
  4. 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;
  5. 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.