Please see description of active comparator arm (arm two).

WSAS-Y is a clinician-rated scale of impaired functioning in school, everyday life, friends and social life, recreation and hobbies and family and close relationships. The scale generates a global score ranging from 0 to 40, with higher scores indicating greater impairment.

The CGAS is a clinician rated single item 1-100 scale that integrates psychological, social, and academic functioning in children as a measure of psychiatric disturbance. Higher values represent better functioning.

The RCADS-25, a shortened version of the Spence Child Anxiety Scale, is a child self-report measure of anxiety- and depression-related psychopathology, with six subscales; separation anxiety, social anxiety, generalized anxiety, panic disorder, obsessive-compulsive disorder and major depressive disorder, as well as a total anxiety-scale. Total range is 0-45, Likert scale ranging from 0-3, with higher values representing a worse outcome.

Beck's Hopelessness Scale is a youth self-rated measure of hopelessness, with 20 dichotomous "true/false" items measuring hopelessness in three domains; feelings about the future, loss of motivation and expectations. A higher score indicates greater difficulties related to hopelessness.

The CHU-9D is a youth-rated measure of health related quality of life for youth, with 9 items on a Likert scale ranging from 0-4. Scores range from 0-36, with a lower score indicating greater health-related quality of life.

Recovering Quality of Life is a parent-rated measure of health-related quality of life for adults, with 10 items on a Likert scale of 0-4, yielding a range of 0-40, where a higher score indicate higher quality of life.

The TIC-P is a 37-item measure assessing healthcare and societal resource use, including for example items on healthcare resource use, medications, school absenteeism, and parental productivity loss. Rated by parents.

LoPF-Q 12-18 is a youth-rated 20-item measure rated on a scale of 0-4, yielding a range of 0-80, where a higher score indicates higher impairment in personality functioning. LoPF-Q is developed for use in adolescent populations between 12 and 18 years to assess the dimensions of personality functioning: identity, self-direction, empathy, and intimacy.

The BSL-Supplement is a youth-rated 9-item measure of self-destructive behaviors under the past 7 days, with answers ranging from "Not at all" to "Daily or even more often". Higher scores indicate a higher frequency of self-destructive behaviors.

CEQ is a youth and parent-rated 3-item measure of treatment credibility and expectancy. Scale points range from 0-9, and 0-100%, where higher scores indicate greater credibility and expectancy.

Participants (parents and youths) are asked to report and rate the discomfort of the eventual adverse events caused by their participation in the treatment. Adverse events are also continuously screened for, recorded, and monitored by study personnel.

At baseline, we use one item from the CEQ addressing treatment expectancy. Scale points for this item range from 0-100%, where a higher score indicates higher expectancy. This item is both youth and parent-rated.

The CSQ is a youth- and parent-rated 8-item measure on a 4-point scale measuring different aspects of satisfaction with treatment, e.g., perception of quality of treatment, if the treatment adequately addressed their needs and overall satisfaction. The score ranges from 8-32, where a higher score indicates higher treatment satisfaction.

The DERS-16 is a youth- and parent-rated measure of difficulties in emotion regulation, with 16 items rated on a 1-5 Likert scale, with scores ranging from 16-80 and higher scores indicating greater difficulties in emotion regulation.

The CERQ is a youth-rated measure of cognitive emotion regulation. The self-blame sub-scale contains 4 items on a Likert scale ranging from 1-5. Scores range from 4-20, and higher scores indicate greater difficulties with self-blame.

The BERQ is a youth-rated measure of behavioral emotion regulation. The sub-scales Seeking Social Support and Actively Approaching contains 8 items on a Likert scale ranging from 1-5. Scores range from 8-40. A higher score indicates greater behavioral emotion regulation regarding seeking social support and actively approaching.

The INQ is a youth-rated measure containing 15 questions rated on a Likert scale of 1-7. The INQ measures beliefs about the extent to which individuals believe their need to belong is met or unmet, as well as the extent to which they perceive themselves to be a burden on the people in their lives. Scores range from 15-105, with higher scores indicating worse outcomes.

The FAD is a 60-item questionnaire that measures an individual's perceptions of his/her family, rated by youth and parents. We utilize a subset of 6 questions from the General Functioning Scale of the FAD. Scores range from 0-18, with higher scores indicating greater general family functioning.

HiPIC-C-30 is a 30-item measure (Likert scale 1-5) assessing the personality constructs of neuroticism, extraversion, openness, conscientiousness, and agreeableness. We will use the subscales measuring neuroticism and conscientiousness, resulting in a 12-item measure, rated by youths and parents. Scores range from 6-30 within each subscale, and higher scores indicate more neuroticism and conscientiousness, respectively.

The CTS is a youth-rated 10-item screening measure of trauma exposure and PTSD symptoms for children ages 6-17. Six questions related to reaction/PTSD symptoms are summed to create a reaction score (rated on a Likert scale of 0-3) yielding a range of 0-18, where a cutoff score of 6 or greater indicates a high likelihood of PTSD diagnosis.

Three self-composed youth-rated questions measuring school functioning, functioning with peers and family functioning. Each question is rated separately and on a Likert scale from 0-11, with a higher score indicating greater functioning in each respective functioning domain.

Subscales from the SSP will be rated by youths and parents, respectively. Youth will answer 12 questions relating to parental and peer support as well as attitudes toward school. Parents will answer 4 questions relating to parental support. Responses are rated on a 0-4 Likert scale. Higher scores indicate greater support and positive attitudes.

The CCNES-A is a measure of parental coping responses in response to youths' negative emotions. The CCNES-A version used in this study consists of nine hypothetical scenarios, accompanied by six theoretically meaningful types of responses (including how well the parent handles their own emotions triggered by the hypothetical situation with the youth) rated by the youth as well as the parent on a Likert scale ranging from 1-7. Higher scores in each subscale representing a response style indicate a higher tendency of the response style.

The PDI-short is parent-rated and includes 2 questions about parenting distress (rated on a Likert scale of 1-5), where higher scores indicate more stress.

The PAS is youth-rated and contains 7 questions on a Likert scale from 0-4 assessing how often aversive parental behaviors have been experienced by the youth. Scores range from 0-28 and higher scores indicate more frequent experiences of aversive parental behaviors.

Data on psychiatric disorders, clinical care consumption, pharmacological prescriptions, employment status, and academic performance will be derived from national population-based registers.

The Parent Self-Efficacy for Suicide Scale is a parent-rated measure of self-efficacy, or confidence, regarding parents' ability to engage in nine activities to prevent or assist their child in managing a suicidal crisis, and parent expectations of adolescents' risk of subsequent suicide. The measure contains 12 items, rated on a scale of 0 (not at all confident) to 10 (completely confident), with an anchor of 5 (somewhat confident), resulting in a range of 0-120. The measure includes two subscales: self-efficacy (9 items) and expectation (3 items). Higher scores indicate higher levels of self-efficacy, and greater expectations of a positive outcome regarding suicidality.

The PHQ-9 is a parent-rated measure used for screening, diagnosing, monitoring and measuring the severity of depression. The measure contains 9 items, rated on a scale of 0 to 3, resulting in a range of 0-27, where a higher score indicates higher levels of depression.