The Danish Youth Cohort: characteristics of participants and non-participants and determinants of attrition.
OBJECTIVE
The aim of this paper is to describe the design and methods used in the Danish Youth Cohort and to give a description of the study participants with special attention to a comparison between participants and non-participants regarding sociodemographic characteristics.


METHODS
A total of 1,945 schools were invited, out of which 506 participated. The participating 7th grades comprised a total of 12,498 responding adolescents. The response rate for the Danish Youth Cohort established in 2005 was 63%. The sample of 12,498 adolescents represents 18.2% of all pupils (n = 68,764) in the 7th grade (mean age: 13.4 years) in Danish schools in 2005. The cohort was followed up in spring 2006 and spring 2007, where the adolescents were in the 8th (mean age: 14.4 years) and 9th (mean age: 15.3 years) grades, respectively.


RESULTS
We found that compared with non-participants the participants were significantly more likely to be girls, to be of Danish ethnicity, and to live in one-family houses. Furthermore, participants more often came from families with two or three children, were more likely to have parents with a high occupational status, parents who were married and parents with a higher total income. Loss to follow-up was only associated with adolescents' higher probability of drinking and use of tobacco, and none of the other factors were associated with attrition.


CONCLUSIONS
The participants in the Danish Youth Cohort represent a great variety of different groups of socio-demographic factors, although they differ from non-participants as regards a range of socio-demographic factors. This should be taken into account in future analyses.
National Institute of Public Health, University of Southern Denmark, Copenhagen, Denmark. mvl@niph.dk
Mathilde
M
Vinther-Larsen
Mette
M
Riegels
Morten Hulvej
MH
Rod
Michaela
M
Schiøtz
Tine
T
Curtis
Morten
M
Grønbaek
D000293:Adolescent; D000294:Adolescent Behavior; D000428:Alcohol Drinking / Q000453:epidemiology; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology / Q000208:ethnology; D015340:Epidemiologic Research Design; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D020407:Internet; D008019:Life Style; D008137:Longitudinal Studies; D008297:Male; D010865:Pilot Projects; D035842:Research Subjects / Q000523:psychology*; D012907:Smoking / Q000453:epidemiology; D012959:Socioeconomic Factors