Here is the design of a clinical trial:

Official Title: Clinical Surveillance Vs. Anticoagulation for Low-risk Patients with Isolated Subsegmental Pulmonary Embolism: a Multicenter Randomized Placebo-controlled Non-inferiority Trial

Included conditions:
- Pulmonary Embolism
- Embolism
- Embolism and Thrombosis
- Lung Diseases
- Cardiovascular Diseases
- Respiratory Tract Diseases
- Venous Thromboembolism
- Anticoagulant-induced Bleeding
- Bleeding

Study Armgroups:
- {'label': 'Anticoagulation', 'type': 'ACTIVE_COMPARATOR', 'description': 'Patients in the anticoagulation group will receive rivaroxaban 15 mg twice daily for the first 21 days, followed by 20 mg once daily for an overall treatment duration of 90 days.', 'interventionNames': ['Drug: Rivaroxaban']}
- {'label': 'No anticoagulation', 'type': 'PLACEBO_COMPARATOR', 'description': 'Patients in the group without anticoagulation will receive placebo twice daily for the first 21 days, followed by one tablet daily for an overall treatment duration of 90 days.', 'interventionNames': ['Drug: Placebo']}

Interventions:
- {'type': 'DRUG', 'name': 'Rivaroxaban', 'description': 'Anticoagulation', 'armGroupLabels': ['Anticoagulation']}
- {'type': 'DRUG', 'name': 'Placebo', 'description': 'Study drug without active agent', 'armGroupLabels': ['No anticoagulation']}

Primary Outcomes:
- {'measure': 'Recurrent venous thromboembolism', 'description': 'Proportion of recurrent, clinically symptomatic, objectively confirmed venous thromboembolism (defined as recurrent fatal or nonfatal pulmonary embolism or lower limb deep vein thrombosis)', 'timeFrame': 'Within 90 days of randomization'}

Please estimate the sample size based on the assumption: 
One-sided type I error of 5%, 80% power, and 5% attrition rate.
Sample size calculation
        Assumptions on VTE recurrence risk are based on data from 127 low-risk patients with isolated SSPE who received anticoagulants (warfarin or low-molecular-weight heparin), showing a VTE recurrence risk of 0.8% at 90 days after diagnosis.23 We chose an absolute non-inferiority margin of 3.5% on the basis of recruitment feasibility, clinical acceptability and previous studies. This corresponds to a difference which is considered acceptable by most physicians and patients for the following reasons. First, our margin is within the range of the 3-month VTE recurrence proportion (0.5%â5%) below which thrombosis specialists would not initiate anticoagulation for PE.61 Second, the definition of a clinically acceptable non-inferiority margin for recurrent VTE must also take into account the potential benefits of withholding anticoagulation, that is, the substantially lower risk of clinically significant bleeding (<1% vs 7% within 3 months for patients receiving anticoagulants).5 23 25 62 Indeed, a patient group with PE who was involved in the trial planning process indicated that given the bleeding risk associated with anticoagulants, a VTE recurrence proportion of <5% seemed acceptable. Finally, similar non-inferiority margins (3%â5%) have been used in key studies comparing different drug treatment regimens and inpatient versus outpatient management for acute VTE.42 44 63â67
        To determine the sample size, we used a Monte-Carlo simulation approach based on an Agresti-Caffo CI for risk difference.68 Assuming a baseline VTE recurrence proportion of 1.0% at 90 days in both treatment groups, an absolute margin of 3.5% defining non-inferiority for clinical surveillance and a sampling ratio of 1:1 allowing 5% attrition (dropouts, including patients who died from non-VTE-related causes) in each group during 90 days, we estimated that 276 patients (138 per group) would result in at least 80% power to establish non-inferiority at an one-sided type I error of 5%.