The Danish Youth Cohort: Characteristics of participants and non-participants and determinants of attrition

AIMS: 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.