The Social Responsiveness Scale (SRS-2) measures deficits in social behavior associated with ASD and can be used to assess the severity of symptoms in ASD. The questionnaire will be completed by multiple raters (parents and teachers). The SRS-2 consists of 65 items scored in a Likert-like scale format ranging from not true=1, sometimes true=2, often true=3 to almost always true =4. It is designed to identify social impairment intrinsic to ASD and to quantify its severity across the duration of the treatment. A total score and five treatment subscale scores (Social Awareness; Social Cognition; Social Communication; Social Motivation; and Restricted Interests and Repetitive Behavior) are obtained. The accepted diagnostic criteria (cut point) for the SRS-2 for the association with a diagnosis of ASD are:<=59 (normal); 60-75 (mild to moderate ASD); and =>76 (severe ASD).

Goal Attainment Scale is a method of scoring the extent to which the patient's individual goals are achieved in the course of the intervention. In effect, each patient has his own outcome measure, but this is scored in a standardized way as to allow statistical analysis. Each goal is rated on a six-point scale, capturing the degree of attainment for each goal area: The present situation is scored -2 (stable). When the patient improves, but achieves less than the expected level this is scored -1 and the expected level is scored 0. When he achieves a better than expected outcome this is scored +1 (somewhat better than the goal) or +2 (much better). Achieving a worse than expected outcome is scored -3 (somewhat worse) In this study, a maximum of 3-4 goals are identified, which are incorporated into a single GAS score.

The Rosenberg Self-Esteem Scale (RSES) [31] will be used to assess self-esteem. It is a widely used 10-item Likert-scale self-esteem measure. Items are answered on a four-point scale - ranging from strongly agree to strongly disagree - measuring positive and negative feelings towards the self. The Dutch version of the RSES is found to be a one-dimensional scale with high internal consistency and congruent validity and a Cronbach's alpha of 0.89

For assessing emotional and/or behavioral problems we will use the CBCL/6-18, completed by parents , the TRF/6-18, completed by teachers, and the YSR/11-18, completed by youths. They include more than 100 items assessing behavioral and emotional problems that are answered on a three-point Likert-type scale (0 = not true, 1 = somewhat or sometimes true, 2 = very true or often true) by parents. The scores will display eight problem scales. The sum of the problem scales 1, 2 and 3 form the 'internalizing behavior' scale, whereas 7 and 8 form 'externalizing behavior'. All subscales together count for the total problem scale. T-scores are computed from raw scores; higher scores on the syndrome scales indicate a greater severity of problems. A T-score of 63 (90th percentile) demarcates the clinical range, which is an indication that a child needs professional help. For the competence scales, lower scores indicate greater severity. A T-score < 37 indicates the clinical range.

For collecting client feedback we will use a brief questionnaires, the Outcome Rating Scale (ORS), which will be easily administered on a regular basis during treatment (https://www.scottdmiller.com/ scholarly-publications-handouts-vitae/).This allows treatment sessions to be evaluated at any time to ascertain whether or not individual treatments are 'on the right track' towards a successful outcome. The ORS is primarily focused on the well-being of the client and is administered at the beginning of the treatment session. The SRS is administered at the end of the session and deals with how the client has experienced the treatment session. The outcomes of the questionnaires are reflected in a graph on an iPad (or on paper when an iPad is not available) per interview to allow the height of the score and progress to be visualized during the sessions.

Family functioning is assessed using a validated questionnaire: the Family Functioning Questionnaire (VGFO, 34 items) (https://www.praktikon.nl/wat-we-doen/vragenlijsten/vgo). The Family Functioning Questionnaire can be answered on a four-point scale ranging from 1 (not applicable) to 4 (completely applicable) with lower scores indicating more problems in family functioning.

For assessing emotional and/or behavioral problems we will use the TRF/6-18, completed by teachers. They include more than 100 items assessing behavioral and emotional problems that are answered on a three-point Likert-type scale (0 = not true, 1 = somewhat or sometimes true, 2 = very true or often true) by parents. The scores will display eight problem scales. The sum of the problem scales 1, 2 and 3 form the 'internalizing behavior' scale, whereas 7 and 8 form 'externalizing behavior'. All subscales together count for the total problem scale. T-scores are computed from raw scores; higher scores on the syndrome scales indicate a greater severity of problems. A T-score of 63 (90th percentile) demarcates the clinical range, which is an indication that a child needs professional help. For the competence scales, lower scores indicate greater severity. A T-score < 37 indicates the clinical range.

For assessing emotional and/or behavioral problems we will use the YSR/11-18, completed by youths. They include more than 100 items assessing behavioral and emotional problems that are answered on a three-point Likert-type scale (0 = not true, 1 = somewhat or sometimes true, 2 = very true or often true) by parents. The scores will display eight problem scales. The sum of the problem scales 1, 2 and 3 form the 'internalizing behavior' scale, whereas 7 and 8 form 'externalizing behavior'. All subscales together count for the total problem scale. T-scores are computed from raw scores; higher scores on the syndrome scales indicate a greater severity of problems. A T-score of 63 (90th percentile) demarcates the clinical range, which is an indication that a child needs professional help. For the competence scales, lower scores indicate greater severity. A T-score < 37 indicates the clinical range.

For collecting client feedback we will use a brief questionnaires, the Session Rating Scale (SRS), which will be easily administered on a regular basis during treatment (https://www.scottdmiller.com/ scholarly-publications-handouts-vitae/).This allows treatment sessions to be evaluated at any time to ascertain whether or not individual treatments are 'on the right track' towards a successful outcome. The ORS is primarily focused on the well-being of the client and is administered at the beginning of the treatment session. The SRS is administered at the end of the session and deals with how the client has experienced the treatment session. The outcomes of the questionnaires are reflected in a graph on an iPad (or on paper when an iPad is not available) per interview to allow the height of the score and progress to be visualized during the sessions.