Family-based Psychoeducation/Relaxation Training (FPRT) as an active control matches the experimental intervention as closely as possible, many elements of the control intervention are similar to FCBT. The main and intended difference between the two approaches is the absence of the Exposure and Response Prevention component, which is deemed the most effective treatment for OCD.

The occurrence of negative adverse events is assessed with the Negative Effects Questionnaire (NEQ). The questionnaire is used for both parental reports and self-reports. It consists of 32 items that are scored on a five-point Likert-scale (0-4) where the highest value represents a severe negative effect of the event. The questionnaire differentiates between negative effects that are attributed to the treatment and those possibly caused by other circumstances, as well as one open-ended question.

This is exploratory outcome showing family factors. Family Accommodation Scale (FAS): a 13-item parent-report measure designed to assess accommodation of a child's OCD-related behaviors over the previous month. In an effort to assess multiple dimensions to accommodations (rather than one in isolation), items are anchored with respect to either frequency or magnitude utilizing a 5-pt Likert scale ranging from 0 (Never) to 4 (Daily) with higher scores indicating greater parental accommodation.

This is exploratory outcome showing family factors. Family Environment Scale (FES)is composed of 90 items and 10 subscales that measure actual, preferred, and expected family social environments. The 10 subscales assess three dimensions: relationship dimensions, personal growth (or goal orientation) dimensions, and system maintenance dimensions. There are three FES forms (real, ideal, and expectations); only the FES real form will be used, which examines a family member's perception of the family as it is. The scale ranges from 10 to 40, with a score of 10 being representative of no family conflict and a score of 40 being representative of the greatest magnitude of family conflict. Therefore, a decrease in score represents a decrease in self-reported family conflict.

This is exploratory outcome showing family factors. Parental Stress Scale (PSS)measures parental stress in the parental relationship and consists of 18 items. The parent answers different statements about his or her relationship to the child by scoring each item on a five-point Likert-scale (1-5), 5 representing the highest amount of agreement with the statement and 1 representing the highest amount of dis-agreement. The positively formulated items are scored in reverse. PSS is thus scored on a scale from 18 to 90, where higher scores reflect greater stress.

This is exploratory clinical outcome.Child Obsessive-Compulsive Impact Scale (COIS) is a 33-item parent-report and self-report questionnaire that assesses OCD-related impairment in various areas of the child's life, including school, social, and home/family activities. For each item, the respondent rates the child's level of impairment on a 4-point Likert scale from 0 (not at all) to 3 (very much). The total score (0-99) is the sum of the item scores. Higher scores reflect more severe impairment.

This is exploratory clinical outcome. Clinical Global Impression Severity and Clinical Global Impression Improvement Scale (CGI-I and CGI-S) measure symptom severity, treatment response, and the efficacy of treatments in treatment studies of patients with mental disorders. Therefore higher scores indicate a worse mental state of the patient. The CGI is rated on a 7-point scale, with the severity of illness scale using a range of responses from 1 (normal) through to 7 (amongst the most severely ill patients). CGI-I scores range from 1 (very much improved) through to 7 (very much worse).

This is exploratory clinical outcome. Occurrence of Serious Adverse Events.Any adverse events such as results in death, is life-threatening, requires hospitalisation or prolongs existing hospitalisation, results in persistent or significant disability or incapacity, or is a congenital anomaly or birth defect, which does not necessarily have a causal relationship with the intervention will be registered register.

This is exploratory clinical outcome. Children's Global Assessment Scale Scale is a numeric scale used by mental health clinicians to rate the general functioning of youths under the age of 18. It is the 100-point single-item score, scored from 0-100 (worse to normal).

This is exploratory clinical outcome. Proportion of patients no longer meeting the diagnostic criteria for OCD, ICD-10 F.42 assessed with Schedule for Affective Disorders and Schizophrenia for school-aged children, Present and Lifetime version (K-SADS-PL) at the end of the intervention (remission).

This is exploratory clinical outcome.This is a binary, exploratory, clinical outcome. Treatment response based on the definition of a reduction in CY-BOCS score at week 16 of at least 30% in intraindividual comparison with the score at week 0.

This is an exploratory clinical outcome. Toronto Obsessive-Compulsive Rating Scale (TOCS) is a quantitative and multidimensional measure of Obsessive-Compulsive traits in children and adolescents. Questionnaire with 21 items, and each item is scored on a 7-point likert scale from -3 to 3. Dimension scores ranges from -3 to 3. The total scale ranges from -63 to 63. Higher values represents worse outcome.

This is exploratory cognition outcome. Risk taking and decision-making assessed by Cambridge Gambling Task

This is exploratory cognition outcome. Planning ability assessed by Stockings of Cambridge test. It is a computerized neuropsychological test assessing executive functions. There are two main outcomes: Reaction time and accuracy. Low reaction time combined with high accuracy represents better planning ability.

This is exploratory cognition outcome. Working memory capacity assessed by Spatial Working Memory test. It requires retention and manipulation of visuospatial information. This self-ordered test has notable executive function demands and provides a measure of strategy as well as working memory errors. Reaction time and accuracy. Low reaction time combined with high accuracy represents better memory capacity.

This is an exploratory cognition outcome. Motor skills assessed with Purdue Pegboard test. It controls for potential effects of differences in motor skills on performance on other tasks in the battery. Test is a rectangular board with 2 sets of 25 holes running vertically and 4 concave cups at the top. Small metal pegs are placed in the cup on the side being tested, with subjects asked to remove the pegs and place them vertically in the holes as rapidly as possible. The number of pegs placed in 30 seconds is scored. More accurate placed pegs represent better motor skills.

This is exploratory cognition outcome. Weather Prediction Learning Task, which is a probabilistic classification learning task in which participants learn to predict the weather (i.e. "sun" or "rain" outcomes) based on combinations of cues that predict "sun" versus "rain" outcomes. Better prediction mean better outcome.

This is an exploratory cognition outcome.The Delis-Kaplan Executive Function System (DKEFS). Test three tests from D-KEFS will be used: Trail Making, Verbal Fluency, and Design Fluency. The three test measures set-shifting ability, processing speed, and response inhibition. Each test has a scaled score going from 1-19, where higher values represent better outcome.

This is exploratory outcome. Thought-Action Fusion Questionnaire for Adolescents (TAFQ-A): a 15-item questionnaire measuring cognitive biases that are thought to play an important role in the development of obsessions. Each item is scored on a 4-point likert scale from 1-4, were higher values represent worse outcome.

This is exploratory outcome. Behaviour Rating Inventory of Executive Function: a self-report and parent-report questionnaire assessing impairment of executive function. Executive function in daily life (range:70-210). Higher score indicate greater executive dysfunction.