(Cost)-Effectiveness of ABFT for Suicidal Youth.

Suicidality is assessed by the Suicide Ideation Questionnaire Junior (SIQ-JR; Reynolds & Mazza, 1999). Scores can range from 0 to 90, with a cut-off score of 31 indicating suicidality.

Assessed with the SCID-5-S, the Structured Clinical Interview for DSM-5 Syndrome Disorders (Michael B. First, JBWW, Lorna Smith Benjamin, Robert L. Spitzer, 2017). The SCID-5-S is designed as a semi structured diagnostic interview for making the major DSM-5 diagnosis. Data obtained here are qualitative, f.i. 'Did you ever wish you were dead or that you would fall asleep and never wake up again? Can you tell me more about that?'.

Assessed with the SCID-5-S, the Structured Clinical Interview for DSM-5 Syndrome Disorders (Michael B. First, JBWW, Lorna Smith Benjamin, Robert L. Spitzer, 2017). The SCID-5-S is designed as a semi structured diagnostic interview for making the major DSM-5 diagnosis. Data obtained here are qualitative, f.i. 'did you ever try to kill yourself? If yes, what did you do? What happened then? Did you try to end your life?'.

Assessed with the SCID-5-S, the Structured Clinical Interview for DSM-5 Syndrome Disorders (Michael B. First, JBWW, Lorna Smith Benjamin, Robert L. Spitzer, 2017). The SCID-5-S is designed as a semi structured diagnostic interview for making the major DSM-5 diagnosis. When 5 or more criteria are scored positive (from A1 - A9), there is an indication for a depressive disorder.

Patient Health Questionnaire 9 (PHQ-9; Kroenke, Spitzer & Williams, 2001). With 0 = 'not at all' and 3 = 'nearly every day'. Scores of 1-4 = minimal depression, scores of 20-27 = severe depression.

Clinical Depression Inventory second edition (CDI-2; Bodden, Braet & Stikkelbroek, 2016). The CDI-2 for parents consists of 17 items, scored on a 4- point likert scale to indicate how often in the past 2 weeks symptoms occurred in their child. They respond to statements like "Looks sad" (0 not at all, 1 sometimes, 2 often, 3 almost always). A CDI-2 cut-off score of 16 is indicative of "significant" depressive symptoms according to the Dutch Mental Health Care guideline (Buitelaar et al., 2009: Multidisciplinaire Richtlijn GGZ, Addendum Depressie bij Jeugd, 2009).

Self-Report Measures of Family Functioning (SRFF; Stark et al., 1990), both the emerging adult and the parent(s)/caregiver(s). The SRFF is composed of 15 five-item factors (e.g., cohesion, conflict, and democratic family style). Items are Likert-type scales rated from 1 (very untrue) to 4 (very true). Items that are scored in reverse are: 2, 4, 5, 8 and 9. A higher score reflects better family functioning.

To investigate whether ABFT improves young adult secure attachment, we will measure their Secure Base Script knowledge (i.e., implicit knowledge about how a care-related interaction unfolds, which participants only acquire if they are securely attached; Waters & Waters, 2006), using the Attachment Script Assessment (ASA; Waters & Waters, 2006; Waters et al., 2019; Waters & Roisman, 2019). The stories narrated by participants will be coded using a 7-point scale (higher scores reflect more SBS knowledge; H. S. Waters & Rodrigues-Doolabh, 2001). A score of 1 = no knowledge about a SBS, a score of 7 = a lot knowledge about a SBS.

Health-related quality of life: 5 Level EQ-5D (EQ-5D-5L; Versteegh et al., 2016). The EuroQOL five dimensions (EQ-5D) is a short questionnaire used to assess utilities at each measurement. Utilities are converted into Quality-Adjusted Life Years (QALYs), using Dutch and Belgian tariffs. Scoring works as followed: for instance for the item 'Mobility', a score of 1 = 'I have no problems in walking about' and a score of 5 = 'I am unable to walk about'. Higher scores on the EQ-5D-5L reflect more perceived problems (i.e. perceived by the patient).

World Health Organization Disability Assessment Schedule 2.0 (WHODAS 2.0; Ustun, Kostanjsek, Chatterji & Rehm, 2010).

Non-suicidal self-injury questionnaire (Buelens, Luyckx, Kiekens, Gandhi, Muehlenkamp & Claes, 2020). This is a self-report 5-item questionnaire that measures non-suicidal self-injury. Items reflect qualitative data of which non suicidal self injury behaviors someone has exhibited.

Autonomy of the young adult, as assessed by the questionnaire 'My Parents and I', a combination of items of the Emotional Autonomy Scale (EAS; Steinberg & Zilverberg, 1986) and the Psychological Separation Inventory (PSI; Hoffman 1984). Items are scored from 1 = 'not true at all' to 5 = 'totally true'. Some items (4, 6, 8, 13, 15) are scored in reverse. A higher score reflects less autonomy.

Entrapment Short-Form Scale (E-SF; De Beurs et al., 2020). The E-SF is a 4-item questionnaire. Respondents are asked to indicate on a 5 point scale (0 = "not at all like me", 1 = "a bit like me", 2 = "moderately like me", 3 = "quite a bit like me", 4 = "extremely like me"), how much each statement applies to the respond. A higher score reflects a worse outcome (so a higher score reflects more entrapment feelings).

Treatment Inventory of Costs in Patients with psychiatric disorders (TiC-P; Hakkaart-Van Roijen, 2002).

Childhood Trauma Questionnaire (CTQ; Garnefski & Kraaij, 2001). This is a self-report 28-item questionnaire that measures 5 types of maltreatment: emotional, physical, and sexual abuse, and emotional and physical neglect. Items that are reverse coded, are: 2, 5, 7, 13, 19, 26, 28. Moderate-severe cutoff scores for each subscale are >= 13 for Emotional Abuse; >= 10 for Physical Abuse; >= 8 for Sexual Abuse; >= 15 for Emotional Neglect; and >= 10 for Physical Neglect.

Working Alliance Inventory (WAI; Horvath & Greenberg, 1986). Measures overall therapeutic alliance and consists of sub scales measuring (quality of therapy) tasks, goals and bonds. Administered to emerging adult and the participating parent(s)/caregiver(s). The total score of the WAI is calculated by adding up all of the items. Reverse items, 4, 8, 12, 16, are scored inversely: (5=1) (4=2) (2=4) (1=5). A higher score represents a stronger alliance.

Treatment fidelity and non-verbal contact will be assessed via scoring the videotapes that ABFT therapists made of their sessions. This will be done by a trained researcher using the The Therapist Behavior Rating Scale-3 (TBRS-3; Diamond, Hogue, Diamond & Siqueland, 1996). Scores are given on a 6 point Likert-scale, with 0 = 'not at all' and 6 = 'thoroughly'. A higher score reflects a better outcome.

Using the Therapy Procedure Checklist (TPC, Weersing, Wijs & Donenberg, 2002), which assesses therapists' reports of the techniques they employ when working with child and adolescent clients, and will be used to monitor Treatment as Usual in both study arms. TPC items encompass the 3 most common therapeutic models for youth: psychodynamic, cognitive, and behavioural.

Demographic characteristics: age, country of birth, current country, country of school, level of education, nationality, ethnicity, marital status, gender (or sex), ethnicity, level of education, gender identity (only applicable to young adults), medication use (only applicable to young adults), substance use (smoking, alcohol, drugs), gambling, psychiatric history (therapy, medication, number of previous episodes, duration of episodes and treatment), psychological help (current and past), number of siblings (only applicable to young adults), current family situation, number of children living at home (only applicable to parent(s)/caregiver(s)), relation to participant (only applicable to parent(s)/caregiver(s)), employment situation, work disability (if not employed), work disability benefit.