Ethics approval and consent to participate
STILL
The study was approved by the Danish Data Protection Agency (ref: 17/12006) prior to collecting data. According to Danish law, questionnaire- and register-based population studies do not require approval by an ethics committee. Still, the National Committee on Health Research Ethics was contacted and they concluded that formal ethics approval was not required (ref: 20182000-83). This study was introduced to students as a study about smoking, health behaviors, and wellbeing. Students were informed that their answers would be used for research purposes only and treated with confidentiality, and that they were able to withdraw from the study at any given time by contacting the research team. Written informed consent was obtained by participants; participants provided written informed consent prior to participating in the surveys by ticking a box in the beginning of the questionnaires. Based on Danish legislation and ethical constraints, Danes aged 15 or older can make an independent decision to participate in surveys without the consent of parents or another legal guardian [