The control condition encompasses treatment as usual comprising seven sessions of specialized treatment, such as individual or group psychotherapy and dietary guidance, provided by interdisciplinary health professionals.

Motivation for change as experienced by the participant post treatment measured with the Stages of Change in Eating Disorders Questionnaire (SOCQ-ED) The SOCQ-ED comprises 13 items measuring motivation to change. For each item, the participant selects among seven different response options which reflects the possible stage of change (i.e. precontemplation, contemplation, preparation, action, maintenance, and termination). Items are rated on a 6-point Likert scale ranging from 1-7, one being the possibility to exclude an irrelevant symptom domain. The higher score the more motivation to change.

Level of identification with the eating disorder and level of embodiment post treatment measured with the Identity and Eating Disorder Questionnaire (IDEA). IDEA is a 23 item self-report questionnaire assessing identification with the eating order and the experience of being (dis)embodied. Items are rated on a 5-point Likert scale ranging from 0 (do not agree) to 4 (strongly agree). The total score is the average of all item ratings. The higher score the more identification with the eating disorder and the less experienced embodiment.

The experience of the eating disorder voice post treatment assessed with the Experience of An Anorexic Voice Questionnaire (EAVE-Q). The EAVE-Q is a self-report questionnaire containing 18 items covering the experience of the eating disorder voice. Each question is scored from 1 (strongly disagree) to 5 (strongly agree). Higher EAVE-Q score is associated with higher symptom load in terms of eating disorder symptoms, depression- and anxiety symptoms, and decreased quality of life.

Level of engagement with the eating disorder voice post treatment measured with the Revised Beliefs About Voices questionnaire (BAVQR), subscale Engagement. BAVQR is a self-report questionnaire with 5 subscales assessing the individual's beliefs about the power and the benevolence/malevolence of the voice and the emotional, and behavioral ways of responding to it. Participants indicate responses on a 4-point Likert scale ranging from disagree (0) to strongly agree (3). The Engagement subscale is divided into emotional and behavioral ways of reacting. The higher score the more emotional and behavioral engagement.

Level of quality of life measured with the Eating Disorder Quality of Life Scale (EDQLS). EDQLS measures 12 domains of eating disorder related quality of life (i.e., disease-specific quality of life). Consists of 40 items, each rated on a 5-point Likert scale from strongly disagree (1) to strongly agree (5). Higher scores indicating better/higher ED-related quality of life.

Perceived difference in power between eating disorder voice and eating disorder patient post treatment measured with the Voice Power Differential Scale (VPDS). VPDS is a self-report questionnaire with seven items measuring the perceived difference in power between the voice and the voice hearer. Power components include power, strength, confidence, respect, ability to cause harm, superiority, and knowledge. Each component is rated on a 5-point scale (1-5). The higher score, the more power the voice is perceived to have compared to the voice hearer.

Level of acceptance and action in relation to the eating disorder voice measured with the Voices Acceptance Scale (VAAS). VAAS is a self-reporting 31 item questionnaire comprising three scales: Acceptance, Action, and a full scale. Responses are provided on a 5-point Likert scale ranging from strongly disagree (0) to strongly agree (4) with higher scores meaning higher levels of acceptance and action in relation to the eating disorder voice.

Level of self-compassion post treatment measured with The Self-Compassion Scale (SCS). SCS is a 26 item self-report measure examining how individuals typically treat themselves when they are struggling. It consists of 6 subscales (Self-Kindness, Self-Judgment, Common Humanity, Isolation, Mindfulness, Over-Identification). Participants indicate, on a 5-point Likert scale ranging from Almost Never (1) to Almost Always (5), how often they think and feel as described. Higher scores indicate greater self-compassion.

Level of body dissatisfaction measured with the Body Shape Questionnaire (BSQ-34) post treatment: BSQ-34 is a 34 item self-report questionnaire measuring body dissatisfaction, including concerns about body shape. Body dissatisfaction is assessed across four dimensions: avoidance and social shame related to body exposure, dissatisfaction with the lower parts of the body, use of laxatives and vomiting to reduce body dissatisfaction, and cognitions and maladaptive behaviors to control weight. Questions are answered on a Likert scale ranging from never (1) to always (6). Higher scores indicate greater concern about body shape and greater body dissatisfaction.

Level of perceived self-efficacy post treatment measured with the General Self-Efficacy Scale (GSE). GSE consists of 10 items rated on a Likert scale ranging from 1 (not at all true) to 4 (exactly true). Higher scores indicate greater self-efficacy.

Ability to regulate emotion post treatment measured with the Emotion Regulation Questionnaire (ERQ). ERQ is a 10 item self-report questionnaire measuring how the respondent controls (regulates and manages) their emotions in two different ways: Emotional experience (cognitive reappraisal) and emotional expression (expression suppression). Items are answered on a 7-point Likert scale ranging from 1 (completely disagree) to 7 (completely agree). The two subscales are scored separately. Higher scores indicate greater use of the specific emotion regulation strategy.

Level of executive functioning post treatment measured with Behavior Rating Inventory of Executive Functioning (BRIEF). BRIEF is a 75 item self-report measure assessing executive functions. Participants indicate whether a specific behavior is a problem on a 3-point Likert scale (0 = Never, 1 = Sometimes, 2 = Often). The global score (General Executive Function) ranges from 0-225. Higher scores indicate more problems with executive functioning.

Participant satisfaction measured with the Client Satisfaction Questionnaire (CSQ). CSQ is an 8 item self-report measure on satisfaction. Responses are given on a 4-point Likert scale ranging from bad (1) to excellent (4). Higher scores indicate higher satisfaction.

Presence of childhood traumas measured with the Childhood Trauma Questionnaire (CTQ). CTQ is a 28 item self-reporting instrument for retrospective assessment of trauma. Answers are given on a 5-point Likert scale ranging from never (1) to very often (5). Higher scores indicates greater severity of childhood trauma.

Level of simulator sickness measured with the Simulator Sickness Questionnaire (SSQ). SSQ is a 16 item self-report measure for simulator sickness. Participants are asked to rate symptoms on a 4-point Likert scale (0-3). The higher score the more symptoms (administered in therapy).

Level of dysfunctional cognitive beliefs measured with the Dysfunctional Attitude Scale questionnaire (DAS-24). DAS-24 is a 24 item self-report measure on dysfunctional cognitive beliefs. Responds are given on a 7-point Likert scale ranging from totally disagree (1) to totally agree (7). Higher scores reflect higher levels of dysfunctional beliefs.

Positive and negative aspects of eating disorders post treatment as perceived by the participant measured with the Pros and Cons of Eating Disorders questionnaire (P-CED). P-CED is a 62 item self-report questionnaire assessing the participants perception of the pros and cons of the eating disorder. Answers are given on a 5-point Likert scale ranging from strongly agree (1) to strongly disagree (5). The sum of the pros and cons respectively are used to form an Ambivalence Score indicating the weight between the two.

Level of personal recovery post treatment measured with the Questionnaire about Process of Recovery (QPR-15)(53). QPR-15 is a 15 item self-report measure for determining the level of personal recovery. Answers are given on a 5-point Likert scale ranging from strongly disagree (0) to strongly agree (4). The higher score the higher level of perceived recovery.

Presence of comorbidity at baseline assessed with the Mini International Neuropsychiatric Interview (MINI). MINI is a structured diagnostic interview for the main psychiatric Axis-I diagnoses in Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) and International Classification of Diseases, Tenth Revision (ICD-10). Participants are asked questions on mental health problems that require a YES or NO answer. Interviewer based rating is done by marking either YES or NO for each symptom within in each diagnosis module. At the end of each module, there are diagnostic boxes indicating whether the diagnostic criteria are met or not.

Analysis of the cost effectiveness of the intervention using the EuroQol Research Foundation self-report questionnaire on health (EQ-5D). EQ-5D asks participants to classify their own health according to five dimensions: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. Each dimension is rated according to three levels: no problems, moderate problems or extreme problems defining 243 potential health states.

Level of cognitive (in)flexibility measured with the Detail and Flexibility Questionnaire in Eating Disorders (Dflex). Dflex is a 24 item self-report questionnaire developed to measure two cognitive thinking styles often reported in patients with an eating disorder: cognitive rigidity (inflexibility) and excessive attention to details. It contains two subscales: Cognitive Rigidity Subscale and Attention to Detail Subscale. Participants respond on a 6-point Likert scale ranging from Strongly Disagree (1) to Strongly Agree (6). The higher the score, the more rigidity/attention to detail.

Monitoring and reporting of adverse and unwanted events from psychotherapy measured with the Negative Effects Questionnaire (NEQ). NEQ is a 32 item self-report questionnaire examining events and outcomes that negatively impact the participant (these can be related to circumstances in the individual's life as well as related to treatment). For each item, the participants indicate whether they experience the statement (yes/no). If yes, the participant rates how negatively the event/outcome affected them on a 5-point Likert scale from 0 (not at all) to 4 (very much). Then it is indicated whether the negative event/outcome is related to treatment or other circumstances. The higher score the more negative effects.

Level of immersion in virtual reality (VR) measured with a modified version of the Multi Modal Presence Scale (MPS). MPS modified version is a 10 item self-report scale measuring the level of immersion in VR, relevant to the dialogue intervention's virtual environment. Answers are given on a 5-point Likert scale ranging from totally disagree (1) to totally agree (5). The higher score the more immersion.