Here is the design of a clinical trial:

Official Title: Short-term Versus Long-term Mentalization-based Therapy for Outpatients With Subthreshold or Diagnosed Borderline Personality Disorder: a Randomized Clinical Trial

Included conditions:
- Borderline Personality Disorder

Study Armgroups:
- {'label': 'Short-term MBT', 'type': 'EXPERIMENTAL', 'description': 'The experimental group is short-term mentalization-based therapy. The treatment program includes 20 weeks of mentalization-based group therapy with conjoined individual therapy every second week. The program also includes psychoeducation and individual caseformulations.', 'interventionNames': ['Other: Short-term MBT']}
- {'label': 'Long-term MBT', 'type': 'ACTIVE_COMPARATOR', 'description': 'The control group is long-term mentalization-based therapy. The treatment program includes 14 months of weekly mentalization-based group therapy with combined individual therapy every second week. The program also includes psychoeducation and individual caseformulations.', 'interventionNames': ['Other: Long-term MBT']}

Interventions:
- {'type': 'OTHER', 'name': 'Short-term MBT', 'description': 'Short-term mentalization-based therapy', 'armGroupLabels': ['Short-term MBT']}
- {'type': 'OTHER', 'name': 'Long-term MBT', 'description': 'Long-term mentalization-based therapy', 'armGroupLabels': ['Long-term MBT']}

Primary Outcomes:
- {'measure': 'Change in severity of borderline personality disorder assessed with the Zanarini Rating Scale for Borderline Personality Disorder (ZAN-BPD) interview', 'description': 'ZAN-BPD is an investigator-administered interview assessing change in severity of borderline personality disorder over time. Each of the nine DSM-5 criteria for borderline personality disorder are rated on a scale from 0-4, where 4 is the most severe, yielding a total score from 0-36.', 'timeFrame': 'Assessed at baseline, and at 8, 16 and 24 months post-randomization'}

Please estimate the sample size based on the assumption: 
Power set at 80% and alpha set at 5% two-tailed.
Sample size
          The sample size was determined by the predicted change in the primary outcome measure, ZAN-BPD. A 3.5-point superiority margin is considered to be the minimal important difference. Consistent with previous trials that have used ZAN-BPD as an outcome measure for a patient group like ours [24, 58], we expect a standard deviation of 8. With power set at 80% and alpha set at 5% two-tailed, a sample size of 83 participants is needed in each treatment group, corresponding to a total of 166 participants.