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Table 4 Baseline health characteristics in 2001, prevalence and relative risk of lost to follow-up in 2004 in Oslo and Hedmark combined

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

Variables collected Base-line Lost to follow-up Relative risk of lost to follow-up
In 2001 N n % RRcrude 1 95% CI RRadj 1,2 95% CI
Self-reported health          
Good/very good 4951 2057 42 Ref    Ref   
Bad/not so good 706 341 48 1.16 1.07 1.26 1.09 0.98 1.20
Total 5657 2398 42       
Mental distress/HSCL above cutt-off          
No 4695 1961 42 Ref    Ref   
Yes 971 415 43 1.02 0.94 1.11 1.06 0.96 1.16
Total 5666 2376 42       
Externalized symptoms          
No 4985 1990 40 Ref    Ref   
Yes 702 399 57 1.42 1.32 1.53 1.18 1.08 1.28
Total 5687 2389 42       
Physical activity          
3+ h 3556 1426 40 Ref    Ref   
0-2 h 2041 913 45 1.12 1.05 1.19 1.03 0.95 1.10
Total 5597 2339 42       
Smoking          
Never or stopped 3962 1538 39 Ref    Ref   
Occasionally or daily 1749 871 50 1.28 1.21 1.36 1.19 1.11 1.28
Total 5711 2409 42       
  1. 1When substituting "lost to follow-up" with "failure to provide DNA" as dependent variable, the RRs were similar
  2. 2Adjusted for invitation group, gender, ethnicity, family economy, parental marital status, educational plans, father's education and income. The number included in adjusted analyses is lower than in crude analyses due to missing data for some of the variables. For example, in analyses of self-reported health there are 5657 in crude analyses and 4527 in adjusted analyses. In particular, we lack data on father's education and income, which are obtained from Statistics Norway. In reanalyses of RRcrude, including only the number available in adjusted analyses, the associations were slightly stronger (except for physical activity), but did not change confidence intervals to significantly/non-significantly for either of the associations.