Methods

Sample Characteristics

The study was conducted at a German university outpatient clinic for psychotherapy and was approved by the local ethics committee for psychology. Hypotheses 1-3, sample size, and statistical analyses were pre-registered on OSF (see https://osf.io/cyzex). Hypothesis 4 was developed later and, therefore, not pre-registered. Data was collected from written archive therapy files at the university CBT outpatient clinic.
To determine the optimal sample size, we conducted an a priori power analysis using G*Power (Faul, Erdfelder, Lang, & Buchner, 2007). Based on trauma prevalence in a prior German CBT study (Trautmann et al., 2019), we assumed a trauma prevalence of 33.3% and calculated sample size to detect a medium effect size of d = 0.5 between trauma groups and for both diagnostic groups separately at the standard alpha error probability of 5% with a power of at least 80%, which yielded an optimal sample size of N = 340.
Following the results of the power analysis, 340 therapy cases were included. All therapies took place between 2011 and 2020. Patients were, on average, 36.97 years old (SD = 13.61) and 64.41% were female. All patients were diagnosed with either a depression disorder, an anxiety disorder, or both by their therapist after at least four diagnostic sessions. On average, patients had 1.54 different psychiatric diagnoses (SD = 0.65, Min = 1, Max = 4), with 46.47% of all patients having at least one comorbidity. Most frequent were depression disorders (86.47%), followed by anxiety disorders (27.94%). Patients diagnosed with PTSD were rare (1.18%). Based on the criteria described below, 50.88% of all patients were classified as having experienced at least one traumatic event and belonged to the trauma group instead of the no-trauma group. Sociodemographic characteristics by trauma group are displayed in Table 1. For a more detailed overview, see Supplemental Material, Table 3, Table 4, and Supplemental Figure 5.
Table 1:
Sociodemographic characteristics of patients at the start of therapy by trauma group.