DBT is a treatment for personality disorders (PD) focused on emotion regulation. It takes at least one group-session per week, and one individual coaching session every two weeks, for the duration of one year.

The OQ-45 is a self-report questionnaire that measures general functioning and physical complaints in the past week. It consists of 45 items; 25 items on psychiatric symptoms and 20 on interpersonal, occupational and social functioning. These are rated from 0 ("Never") to 4 ("Almost always"). Some items need to be reversed in scoring. There are four subscales: Symptomatic Distress (25 items); Interpersonal Relationships (11 items) and Social Role (9 items). A higher score indicates worse functioning.

The BDI is a 21-item self-report questionnaire assessing the severity of depression. Each item consists of four statements indicating different levels of severity of a particular symptom experienced over the past week, ranging from low (0) to high (3) severity. Scores for all 21 items are summed to yield a single depression, with a maximum depression score of 63.

The AUDIT is a self-report questionnaire, with 10 items about alcohol use. These range from 0 ("Never") to 4 ("Daily"). The items are summed to create a total score from 0 (no alcohol risk) to 40 (maximum alcohol risk). The first three questions are about alcohol consumption, question 4-6 about alcohol dependency and question 7-10 about alcohol related problems.

The SCID-5-S is a clinician administered semistructured clinical interview to assess DSM-5 disorders (but not personality disorders). It consists of in total 14 modules, comparable to the DSM-5. Patients are assessed on the disorder, resulting in a score of absent/present for each disorder. In this study, we use the modules current depressive episode, current manic episode, current persistent depressive disorder, delusions & hallucinations, alcohol abuse, substance abuse, panic disorder, agoraphobia, specific phobia, generalized anxiety disorder, obsessive-compulsive disorder, current anorexia, bulimia and binge eating disorder.

WHODAS 2.0 is a 36-item self-report questionnaire assessing the daily function of activity and participation within the 30 previous days, including the following six domains: Cognition, Mobility, Self-care, Getting along, Life activities and Participation. The responses on each item range from no difficulty (1) to extreme difficulty (5). Responses to the six dimensions are weighted and summed to create a total score between 0 (no disability) and 100 (complete disability).

The EQ-5D-5L measures health-related quality of life and consists of five dimensions: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. Each dimension has 5 levels: no problems, slight problems, moderate problems, severe problems and extreme problems. The answer to each item results in a 1-digit number that expresses the level selected for that dimension. The digits for the five dimensions can be combined into a 5-digit number that describes the patient's health state, with higher numbers indicating more severe problems.

TiC-P is a self-report questionnaire assessing direct medical costs and productivity costs due to absence from work or reduced efficiency during work in patients with a mental disorder. The first part of the TiC-P includes 14 structured yes or no questions on the use of medical resources, each followed by a question on the volume of medical consumption. The second part includes five items on work absence, reduced efficiency at work and related productivity losses.

The NSSI screener consists of 7 multiple-choice items assessing non-suicidal self-injury. In case of an affirmative responses to the item 'Have you ever done any of the following with the purpose of intentionally hurting yourself?' engagement in NSSI is determined.

The Personality Assessment Inventory-Borderline Features (PAI-BOR) Scale is a self-report measure assessing the presence and severity of BPD. The BAI-BOR consists of four subscales of six items each, reflecting four main characteristics of BPD: affective instability, negative relationships, identity problems and self-harm. Each items is rated on a four-point scale, ranging from false (0) to very true (3). A total PAI-BOR score of 38 or more indicates the presence of significant BPD features, whereas a score of 60 or more indicates typical borderline personality functioning.

The State-Trait Anger Scale (STAS) is a 20-item questionnaire assessing state and trait anger. Both types of anger are assessed through 10 items, that are scored on a four-point rating scale, ranging from not at all (0) to very much so (4). The STAS has acceptable to strong internal consistency (Kroner & Reddon, 1992).

The Difficulties in Emotion Regulation Scale (DERS) is a 41-item self-report measure to assess clinically relevant difficulties in emotion regulation. DERS items reflect difficulties within four dimensions of emotion regulation: awareness and understanding of emotions, acceptance of emotions, the ability to engage in goal-directed behaviour and to refrain from impulsive behaviour, and access to effective emotion regulation strategies. Items are scored on a five-point scale, ranging from almost never (1) to almost always (5). Although the Dutch version of DERS (DERS-NL) has not been validated yet, the original DERS showed high internal consistency, good test-retest reliability and adequate construct and predictive validity (Gratz & Roemer, 2004).