Aim: To present the study design, study population and questionnaire content of the Danish National Youth Study 2019, and to describe the differences between participants and non-participants regarding demographic and socioeconomic characteristics. Method: The Danish National Youth Study 2019 is a nationwide web-survey among high school students in Denmark. Data was collected from January to April 2019 through a self-administered questionnaire in the classroom. The questionnaire included 120 questions on, for example, physical and mental health, health behaviour and well-being. Data collection took place at 50 general high schools, 32 preparatory high schools, 15 commercial high school and 19 technical high schools. Results: A total of 29,086 students participated (20,287 general high school students, 2,113 preparatory higher school students, 4027 commercial high school students and 2659 technical high school students) corresponding to 66% of the students in the 88 participating schools (31% of invited schools). Among students, 55% were female and the mean age was 17.8 years. Participants were more likely to be female, to be younger, to be of Danish origin, and have family disposable income in the highest quartile compared to non-participants. Conclusions: The Danish National Youth Study 2019 contributes to knowledge on high school students’ health, health behaviour and well-being that can support health planning and prioritizing, through identification of specific risk groups at both local and national level. The study also offers great opportunities for future research as it provides possibility of linkage to various Danish national registers.

Background Recruiting young people for health and intervention studies by traditional methods has become increasingly challenging. The widespread access to the internet may offer new strategies for online recruitment. Objective This study aims to assess the feasibility of online recruitment for a randomized controlled trial evaluating the effectiveness of Mindhelper, an online national youth mental health promotion service. The target group was young Danes aged 15-25 in need of mental health promotion. Methods Advertisements for recruitment were set up on Facebook and Instagram. Browser history was collected for a subsample of participants. We compared basic characteristics of participants who completed the baseline survey and those who did not, as well as of participants who completed the follow-up survey and those who were lost to follow-up. The significance of these differences was tested with the Pearson chi-square test. Results A total of 560 Danes aged 15-25 were recruited within 1 month (ie, had completed the baseline survey). Among these participants, 356 (63.6%) were at risk of developing depression or stress. The average advertisement price per participant completing the baseline questionnaire was 31 DKK (approximately €4 [US $4.2]). The follow-up survey was sent to 545 participants, of whom 318 (58.3%) completed the survey. No statistically significant differences were observed in baseline characteristics of participants who completed the follow-up and those who were lost to follow-up in terms of gender (P=.45), age (P=.35), occupation (P=.17), cohabitation (P=.90), mental well-being (P=.26), mental illness (P=.44; impact of the illness, P=.05), or use of the internet when having a hard time (P=.92). Conclusions We conclude that it is feasible to recruit young Danes online for a large-scale randomized controlled trial assessing the effectiveness of Mindhelper. Trial Registration ClinicalTrials.gov NCT04650906; https://clinicaltrials.gov/ct2/show/NCT04650906

BACKGROUND Recruiting young people for health and intervention studies by traditional methods has become increasingly challenging. The widespread access to the internet may offer new strategies for online recruitment. OBJECTIVE This study aims to assess the feasibility of online recruitment for a randomized controlled trial evaluating the effectiveness of Mindhelper, an online national youth mental health promotion service. The target group was young Danes aged 15-25 in need of mental health promotion. METHODS Advertisements for recruitment were set up on Facebook and Instagram. Browser history was collected for a subsample of participants. We compared basic characteristics of participants who completed the baseline survey and those who did not, as well as of participants who completed the follow-up survey and those who were lost to follow-up. The significance of these differences was tested with the Pearson chi-square test. RESULTS A total of 560 Danes aged 15-25 were recruited within 1 month (ie, had completed the baseline survey). Among these participants, 356 (63.6%) were at risk of developing depression or stress. The average advertisement price per participant completing the baseline questionnaire was 31 DKK (approximately €4 [US $4.2]). The follow-up survey was sent to 545 participants, of whom 318 (58.3%) completed the survey. No statistically significant differences were observed in baseline characteristics of participants who completed the follow-up and those who were lost to follow-up in terms of gender (<i>P</i>=.45), age (<i>P</i>=.35), occupation (<i>P</i>=.17), cohabitation (<i>P</i>=.90), mental well-being (<i>P</i>=.26), mental illness (<i>P</i>=.44; impact of the illness, <i>P</i>=.05), or use of the internet when having a hard time (<i>P</i>=.92). CONCLUSIONS We conclude that it is feasible to recruit young Danes online for a large-scale randomized controlled trial assessing the effectiveness of Mindhelper. CLINICALTRIAL ClinicalTrials.gov NCT04650906; https://clinicaltrials.gov/ct2/show/NCT04650906