*2.2. Materials*

ME/CFS symptoms were assessed with the De Paul Symptom Questionnaire Short Form (DSQ-SF, 14 items) [31] and the DePaul Post-Exertional Malaise Questionnaire (DSQ-PEM; eight out of 10 items assessed; due to a programming error two items identical to the DSQ-SF were not assessed) [33]. Functional status was assessed with the Short-Form Health Survey (SF-36; 36 items) [35,36]. Furthermore, to assess access to medical care, participants were asked "Did you utilize any of these services in the past 6 months in regard to your ME/CFS? Primary care physician (GP), ME/CFS-specialist, neurologist, other specialist, hospital/stationary care, ME/CFS self-help, mental health, alternative medicine" and "Are there any services that you would like to use but are not accessible to you for one or more of the following reasons? Financial/insurance reasons, lack of knowledge of service availability (who treats my disease?), ME/CFS-associated impairment prevented access to service, travel distance and lack of transportation, no ME/CFS-specialist in geographic area, ME/CFS-specialist is not covered by health insurance, ME/CFS-specialist has a full waiting-list", adapted from [20,22] to the characteristics of the German health-care system. Patient satisfaction with medical care was assessed with nine items ("Please indicate how satisfied you are with the care by your doctor that you are visiting most frequently because of ME/CFS", e.g., "Overall, I feel satisfied with my appointments", "Knowledgeable about symptoms/course of ME/CFS", *1* = strongly disagree, *4* = strongly agree) [20]. In addition, participants indicated whether the doctor is a generalized or specialized physician (further indicating the area of specialty). Finally, participants completed items on demographics and illness history from the DSQ-2 (items 3–11; 94–99; 111–115, 116; [30]; demographics adapted to the German context).

#### *2.3. Statistical Analyses*

Statistical analyses were conducted with IBM SPSS version 26 and Mplus version 7 (confirmatory factor analysis only). The level of significance was α < 0.05, confidence intervals are displayed at the 95% level. Sample characteristics, health-related demographics and medical care access were investigated with descriptive statistics and frequency analysis. Multi-item measures (i.e., satisfaction with medical care, DSQ-SF) were aggregated to scales, as internal consistency was sufficient (Cronbach's α > 0.80). Analyses of means was conducted with one-sample *t*-tests and paired-samples *t*-tests with bootstrapping (1000 samples). To investigate the factor structure of the DSQ-SF, we conducted the confirmatory factor analysis. Cutoffs for model fit statistics were CFI/TLI ≥ 0.90, RSMEA ≤ 0.08, and SRMR ≤ 0.05. Validity was investigated with correlational analyses, effect sizes were interpreted in accordance with Cohen (small effect: *r* = 0.10, moderate effect: *r* = 0.30, large effect: *r* = 0.50; [37]).
