Efficacy of a Transdiagnostic Online Prevention Approach in Adolescents (EMPATIA)

Assessed with a structured interview ("Global Functioning Social"; GFsocial; Cornblatt et al., 2007). Scores range from 1 to 10 with higher scores indicating higher social functioning.

Assessed with a structured interview ("Global Functioning Role"; GFrole; Cornblatt et al., 2007). Scores range from 1 to 10 with higher scores indicating higher role functioning.

Assessed with the Kinder-DIPS ("Diagnostic Interview for Mental Disorders in Children and Adolescents"; Schneider, Pflug, In-Albon, \& Margraf, 2017)

Assessed with the German version of the "Client Sociodemographic and Service Receipt Inventory" (CSSRI-EU; Chisholm et al., 2000; Roick et al., 2001).

The "Patient Health Questionnaire-9 for Adolescents" (PHQ-A; Johnson et al., 2002) will be used to assess depressive symptoms. This self-report questionnaire consists of 9 items and higher scores indicate higher depressive symptoms.

The "Altman Self-Rating Mania" Scale (ASRM) will be used to assess hypomania/bipolar symptoms. It is a 5-item Likert scale in which manic symptoms, corresponding to diagnostic criteria for mania, are rated from 0 to 4 according to increasing severity.

The GAD-7 will be used to measure general anxiety symptoms with 7 items. Higher scores indicate higher levels of anxiety.

The "Short Obsessive-Compulsive Disorder Screener" (SOCS), a 6 item screening instrument, will be used to assess obsessive-compulsive symptoms.

The 15 item self-report questionnaire CAPE-P15 (Capra et al., 2013) assesses three domains: paranoid ideation, bizarre experiences, and perceptual anomalies. Scores can range from 15 to 75. Higher scores indicate higher severity of psychotic-like experiences.

The CRAFFTd (Tossman et al., 2009) is a one-dimensional screening tool consisting of 6 questions. It will be used to screen for problematic substance consumption in adolescents. Higher scores indicate greater problematic substance consumption.

The "Child Eating Disorder Examination-Questionnaire" (ChEDE-Q8) used in this study is an 8 item self-report scale to assess specific eating disorder psychopathology.

The subscale of the "Strengths and Difficulties Questionnaire" (SDQ-s; Becker et al., 2018) will be used to assess subclinical conduct problems. 5 items, higher scores indicate higher severity of conduct problems.

(Kinder-DIPS, Additional-Module NSSV; Margraf et al., 2017; Schneider et al., 2017). The suggested 7 questions by Margraf et al. (2017) and Schneider et al. (2017) will be assessed via self-report. Non-suicidal self-injurious behavior in the past year on five or more separate occasions are considered as subclinical.

The "Somatic Symptom Scale" (SSS-8) with 8-item will be used for the assessment of somatic symptom burden. Higher scores indicate higher somatic symptom burden.

"Perceived Stress Scale" to assess current levels of stress (PSS-4), 4-item version with responses scored on a 1- to 5-point scale.

The WSAS-Y ("Work and Social Adjustment Scale-Youth") consists of five items that are rated on a 9-point Likert scale, generating a global score ranging from 0 to 40 (Jassi et al., 2020).

The "Checklist of Avoidance Strategy Engagement for Adolescents" (CEASE-A) is a psychometrically sound measure of avoidance (Kennedy, 2015). The 29-item checklist assesses frequency of engagement in avoidance behaviors to manage or avoid feelings of anxiety, anger, fear, or sadness on a 5-point Likert-type scale.

The "Frost Multi-Dimensional Perfectionism Scale-Brief" (F-MPS-B; Burgess et al. 2016) is a brief 8-item version of the original F-MPS (35-items) (Frost et al. 1990). Items are rated on a 5-point Likert type scale with higher scores indicating greater perfectionism.

The "Repetitive Thinking Questionnaire" (RTQ-10) is a 10-item scale to measure the common transdiagnostic factor repetitive negative thinking. Items are rated on a 5-point scale.

The "Difficulties in Emotion Regulation Scale" (DERS-16) is a 16-item self- report measure of emotion regulation difficulties (Bjureberg et al., 2016).

The "Intolerance of Uncertainty Scale" is a 12-item short-form of the original 27-item Intolerance of Uncertainty Scale (Freeston et al., 1994). Items are scored on a 5-point Likert scale ranging from 1 to 5.

The "Children's Rejection Sensitivity Questionnaire" (C-RSQ; Downey et al., 1998) assesses the rejection sensitivity through 6 hypothetical interpersonal situations with ambiguous outcomes. Participants indicate on a 6-point Likert scale whether they would be nervous about the situation, how angry they would be about the situation, and what their outcome expectancy is.

The ZUF-8 is a questionnaire with 8 items for the global, one-dimensional assessment of patient satisfaction (Schmidt et al., 1989).

Assessed within the online-intervention. Indicator of adherence: time spent in program.

Assessed within the online-intervention. Indicator of adherence: number of clicks.

Assessed within the online-intervention. Indicator of adherence: number of modules completed.

Assessed within the online-intervention. Indicator of adherence: number of exercises completed.

Negative effects of the intervention will be assessed using a version of the "Inventory for the Assessment of Negative Effects in Psychotherapy" (Kinder-INEP; Bieda et al., 2018; INEP; Ladwig et al., 2014) adapted for online interventions and for children and adolescents. 12 Items.

The "Working Alliance Inventory for guided Internet Interventions" (WAI-I; Gómez Penedo et al., 2020) is a self-report questionnaire that measures therapeutic alliance in online interventions with therapeutic support. "Working Alliance Inventory for Children and Adolescents" (WAI-CA) consists of 12 items and has good psychometric properties (Gómez Penedo et al., 2020).

The "Motivation for Youth's Treatment Scale" (MYTS; Breda \& Riemer, 2012) measures youth's motivation for therapy through a self-report questionnaire. The MYTS is a psychometrically based scale consisting of 8 items. Responses are rated on a 5-point likert scale. Higher ratings indicate higher motivation.

Assessed by the Kinder-DIPS interview (Margraf et al., 2017; Schneider et al., 2017).

Suicidality will throughout the study be evaluated by the item 9 of the "Patient Health Questionnaire-9 for Adolescents" (i.e., PHQ-A; Johnson et al., 2002). With a score "3" as the highest score.