Table 1.

Overview of assessments and assessment points during the SOPHIE study.

Construct		Description of assessment	Timeline of assessment
			Baseline	Week 2	Midintervention or control at week 4	Week 6	Postintervention or control at week 8	Follow-up at week 20
Sociodemographic variables
	Demographic information	During the first telephone interview, participants are asked about their gender, age, nationality, native language, and living as well as family situation.	✓
	Socioeconomic status	During the first telephone interview, the Family Affluence Scale with 6 items is assessed as an indicator for socioeconomic status and demonstrates good psychometric properties to detect socioeconomic differences in Western European countries [68,69].	✓
Primary outcome
	Social anxiety	The Social Phobia Inventory (SPIN) is a self-report questionnaire assessing 3 dimensions of social anxiety: fear, avoidance, and physiological symptoms. In 17 items, adolescents rate the frequency of each symptom on a 5-point Likert scale from not at all (0) to extremely (4) ([61], German version by [62]). The SPIN is a reliable self-report measure with good psychometric properties in adolescents [70,71]. Cut-off for subclinical social anxiety symptoms is set at 16 [63].	✓		✓		✓	✓
Secondary outcomes
	Mental disorders	The structured clinical interview for mental disorders across the life span is a structured clinical interview to assess the presence of SADa and comorbid past and present mental disorders according to the Diagnostic and Statistical Manual of Mental Disorders-5 ([72], German Version of Kinder-DIPS; [73]). In this study, the full interview is conducted by phone at baseline. At post- and follow-up assessment, only the section for SAD and sections of previously met diagnoses are assessed.	✓				✓	✓
	Global functioning	The Global Functioning Social and Role Scale is a structured interview that is also carried out via phone in this study [74]. Two subscales (social and role) allow for an assessment of the level of functioning (concurrently and in the past year) on a scale ranging from 1 to 10 with 10 representing the highest level.	✓				✓	✓
	Utilization of help	The Client Socio-Demographic and Service Receipt Inventory [75,76] is a standardized interview to assess the utilization of help and help seeking behavior. The applied version has already been tested in adolescents [76] and is administered during the postintervention and follow-up telephone interview.					✓	✓
	Fear and avoidance	The Social Anxiety Scale for Adolescents measures fear of negative evaluation and social avoidance in adolescents with 18 items answered on a 5-point Likert scale from not at all (1) to all the time (5) [77]. Validity and reliability were tested in both clinical and nonclinical samples [77,78].	✓		✓		✓	✓
	Anxiety	The Generalized Anxiety Disorder 7 [79] questionnaire measures self-reported frequency of anxiety symptoms with 7 items answered on a 4-point Likert scale from not at all (0) to nearly every day (3). The German questionnaire produced good psychometric properties in adolescents [79,80].	✓		✓		✓	✓
	Depression	The Patient Health Questionnaire-9 for Adolescents [81] assesses depressive symptomatology with 9 items asking about the frequency of symptoms over the last 2 weeks on a 4-point Likert scale from not at all (0) to nearly every day (3). The self-report questionnaire has good psychometric properties for depression detection in adolescents [82].	✓		✓		✓	✓
	Quality of life	The KIDSCREEN-10 is used to measure health-related quality of life with 10 items answered on a 5-point Likert scale and has good psychometric properties [83].	✓		✓		✓	✓
	Self-esteem	The Rosenberg Self-Esteem Scale [84] assesses self-esteem in 10 items on a Guttman scale. This questionnaire is applicable to different age groups; in this study, the adolescent version is applied.	✓		✓		✓	✓
	Therapy expectancy	The Credibility or Expectancy Questionnaire [85] assesses treatment expectancy and credibility with 6 items. There are 2 rating scales, 1 from not at all (1) to very (9) and another from 0 to 100%. For this study, the wording of the items was adapted to the target group of adolescents.	✓
	Therapy motivation	The Motivation for Youth’s Treatment Scale (MYTS) [86] measures adolescent’s motivation for therapy through a self-report questionnaire. Eight items are rated on a 5-point Likert scale from strongly disagree (1) to strongly agree (5). All adolescents complete this assessment at baseline, only adolescents randomized to the intervention group complete the MYTS at week 2, 4, 6, and 8.	✓	(✓)b	(✓)	(✓)	(✓)
	Negative effects of intervention	The Inventory for assessment of negative effects in psychotherapy (German: Inventar zur Erfassung Negativer Effekte in der Psychotherapy, Kinder-INEP; [87,88]) has been adjusted to the online format of the intervention, reducing the questionnaire by 6 items to 15 items. This questionnaire is only answered by adolescents in the intervention group.					(✓)
	Satisfaction with the intervention	The ZUF-8 is an 8-item questionnaire for global, unidimensional assessment of patient satisfaction [89]. The questionnaire was adapted accordingly to measure satisfaction with the online intervention. Adolescents randomized to the intervention answer questions on a 4-point Likert scale from 1 to 4.					(✓)
	Therapeutic alliance	The Working Alliance Inventory for guided Internet Interventions (WAI-I; [90]) is a self-report questionnaire that measures therapeutic alliance in online interventions with therapeutic support. The 12 items rated on a 5-point Likert scale from rarely (1) to always (5) were adapted to adolescents. Only adolescents in the intervention group are assessed with the WAI-I.		✓	✓	✓	✓
	Adherence to the online intervention	Adherence is operationalized through the extent to which the online intervention is used. The number of finished modules, the number of completed exercises, and the time spent in the online intervention are recorded automatically. As in other studies of online interventions, adherence is calculated with respect to each of these variables and with respect to a composite measure [91].					(✓)

^aSAD: social anxiety disorder.

^bIndicated time points in parentheses reflect assessments only administered in the intervention group.