Here is the design of a clinical trial:

Official Title: Treatment Effects of Family Based Cognitive Therapy in Children and Adolescents With Obsessive Compulsive Disorder

Included conditions:
- Obsessive-Compulsive Disorder in Children
- Obsessive-Compulsive Disorder in Adolescence

Study Armgroups:
- {'label': 'FCBT', 'type': 'EXPERIMENTAL', 'description': 'The Family Based Cognitive Behavioural Therapy (FCBT)', 'interventionNames': ['Behavioral: Family Based Cognitive Behavioural Therapy']}
- {'label': 'FPRT', 'type': 'ACTIVE_COMPARATOR', 'description': 'Family-based Psychoeducation /Relaxation Training (FPRT)', 'interventionNames': ['Behavioral: Family Based Psychoeducation/Relaxation Training']}

Interventions:
- {'type': 'BEHAVIORAL', 'name': 'Family Based Cognitive Behavioural Therapy', 'description': 'Family Based Cognitive Behavioural Therapy (FCBT) focuses on the interrelation between thought, emotion, and behaviour, Exposure and Response Prevention, family involvement, homework assignments, and formulating of specific goals for the child. The important, active components is Exposure and Response Prevention. It involves exposing the child to a feared object, situation or thought, and preventing the child from carrying out compulsions to show the child that distress/anxiety can decrease or disappear without performing rituals.', 'armGroupLabels': ['FCBT']}
- {'type': 'BEHAVIORAL', 'name': 'Family Based Psychoeducation/Relaxation Training', 'description': '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.', 'armGroupLabels': ['FPRT']}

Primary Outcomes:
- {'measure': "Change in Children's Yale-Brown Obsessive-Compulsive Scale (CY-BOCS) score", 'description': "Children's Yale-Brown Obsessive-Compulsive Scale (CY-BOCS) is a 10-item questionnaire assessing the severity of obsessive compulsive disorder (OCD). Severity of compulsions and obsessions are rated on a scale from 0 (none) to 4 (extreme). The total score is calculated by summing the 10 individual scores and ranges from 0 to 40, where higher scores indicate more extreme symptoms.", 'timeFrame': 'Week 0, 4, 8, 16 and 40'}

Please estimate the sample size based on the assumption: 
Using a power of 80%, a two-sided alpha of 5%, and expecting a standard deviation (SD) of 8 on the CY-BOCS total score. To account for drop-outs and missing data, the power is increased to 85.7% by including up to 20 extra participants.
Sample size estimation and feasibility of recruitment
          The sample size is based on the primary outcome, the CY-BOCS score (continuous variable) measuring severity of OCD symptoms on 10 items which can be rated 0 to 4 points (total score range 0 to 40). Using a power of 80%, a two-sided alpha of 5%, and expecting a SD of 8 on the CY-BOCS total score based on reports in similar patient groups [13], the required sample size necessary to detect or reject a minimal relevant difference of at least 4 points on CY-BOCS total score was estimated to be 64 participants in each intervention group, a total of 128 [13, 47]. Power calculations for secondary outcomes (KIDSCREEN-52 and NEQ) will follow in a detailed statistical analysis plan (see below).
          To estimate the expected recruitment potential of OCD patients in CAMHC we drew on the available hospital statistics in the planning phase of the trial before initiation in 2018. In the year 2016, 108 patients aged 8â17âyears were referred to and treated in CAMHC for OCD. We therefore estimated that around 324 patients would be eligible for participation in the TECTO trial within our recruitment period of 3 years (ultimo 2018 to ultimo 2021). With a target sample size of 128, we considered it feasible to recruit 40% of all referred patients. Randomised clinical trials with psychiatric patients are prone to drop-outs and missing data [48]. Thus, we aim to include and randomise up to 20 extra participants, i.e. up to 148 participants in total (74 in each group), which will increase our power for our primary outcome to 85.7%.