Micro-choice-based Intervention for Multicausal Fatigue

defined as the percentage of participants who complete all phases of the study. To assess this, the investigators will evaluate:

• Dropout rates at different phases of the study: preparation, concentrated intervention and follow up
• Completion rates within each phase to identify potential barriers to adherence and engagement

Patient satisfaction will be assessed using the ZUF-8 ("Fragebogen zur Messung der Patientenzufriedenheit": the validated German version of Client Satisfaction Questionnaire). Satisfaction will be defined as a mean total score of ≥20, with no individual item scoring ≤2. This cut-off was selected based on prior research, in which scores ≥20 were interpreted as indicating "good" satisfaction.

Fatigue is assessed by the 11-item Chalder Fatigue Scale (46), e.g. by asking "Do you have difficulty concentrating?". Items are answered on a 4-point scale ranging from 0 = "Better than usual" to 3 = "much worse than usual".

The Patient Health Questionnaire (PHQ-15; 34) assesses somatic symptom severity during the past 4 weeks. Patients are asked how impaired they felt by their symptoms, e.g. back pain. Items are answered on a 3-point scale ranging from 0 = "not bothered at all" to 2 = "bothered a lot".

The Somatic Symptom Disorder Questionnaire (SSD-12; 35) assesses psychological features of somatic disorders, e.g. catastrophizing thoughts and health anxiety, e.g. with the statement "I think that my physical symptoms are signs of a serious illness". Items are answered on a 5-point scale ranging from 0 = "never" to 4 = "very often".

The 8-item Patient Health Questionnaire Depression Scale (PHQ-8; 36) assesses the severity of depressive disorders. Patients are asked if in the past weeks, they were bothered by a problem, e.g. by having little interest or pleasure in doing things. Items are answered on a 4-point scale ranging from 0 = "not at all" to 3 = "nearly every day".

Anxiety is assessed with the Generalized Anxiety Disorder Questionnaire (GAD-7; 44). It assesses if the patient was bothered by complaints related to anxiety during the past 2 weeks, e.g. by asking about having trouble to relax. Items are answered on a 4-point Likert scale ranging from 0 = "not at all" to 3 = "nearly every day".

The World Health Organization Disability Assessment Schedule (WHODAS 2.0) assesses and classifies disability due to health problems during the past 4 weeks. This study will utilize the 12-item version. An example item would be "In the past 4 weeks, how much difficulty did you have in walking a long distance such as a kilometer (or equivalent)?". Items are answered on a 5-point scale ranging from 0 = "none" to 4 = "extreme or cannot do".

As a measure of life quality, the EuroHIS Quality-of-Life-8 (QOL-8; 42) will be used. This 8-item instrument assesses quality of life and perceived health during the past 4 weeks, e.g. by asking "Do you have enough energy for everyday life?". Items are answered on a 5-point scale, with wording differing between questions (e.g., from "very dissatisfied" to "very satisfied").

assessed via questionnaires: at full work, at partial work, not at work.

Work and Social Adjustment Scale (WSAS;(38) ) assesses functional impairment attributable to a health condition in key areas of daily life, including work, home management, social and private leisure activities, and close relationships. An example item is "Because of my illness, my ability to work is impaired." Items are rated on a 9-point scale ranging from 0 = "not at all impaired" to 8 = "very severely impaired". Higher total scores indicate greater functional impairment.