Here is the design of a clinical trial:

Official Title: Study Protocol for a Pilot Randomized Controlled Trial on a Smartphone Application-based Intervention for Subthreshold Depression

Included conditions:
- Subthreshold Depression

Study Armgroups:
- {'label': 'Smartphone Application', 'type': 'EXPERIMENTAL', 'description': 'Participants watch a video using the Smartphone Application displaying positive word stimuli.', 'interventionNames': ['Device: Smartphone Application']}
- {'label': 'No Intervention', 'type': 'NO_INTERVENTION', 'description': 'No Intervention.'}

Interventions:
- {'type': 'DEVICE', 'name': 'Smartphone Application', 'description': 'The experimental group will watch movies using the Smartphone application for at least 10 min a day for 5 weeks.', 'armGroupLabels': ['Smartphone Application']}

Primary Outcomes:
- {'measure': 'Depression severity as measured by the Center for Epidemiologic Studies Depression Scale (CES-D) score.', 'description': 'The CES-D is a 20-item self-report questionnaire used to measure depressive symptoms. The CES-D is a four-point Likert scale, with each item scored from 0 to 3. The total score ranges from 0 to 60 points. The higher the score, the stronger the depressive symptoms.', 'timeFrame': 'Change from Baseline CES-D at 5 weeks'}

Please estimate the sample size based on the assumption: 
A pilot RCT does not require sample size calculations using significance level, power, difference between groups, and SD. However, 15 to 20 participants per group are required to ensure scientific validity.
3.7
        Sample size
        Four factors are required to calculate the sample size: significance level, power, difference between groups, and standard deviation (SD).[35] However, considering that no previous 2-arm trial has used the SPSRS application for individuals with StD, values for the difference between groups and SD remain unknown. Although a pilot RCT does not require sample size calculations using the aforementioned 4 factors,[26] 15 to 20 participants per group are required to ensure scientific validity of the pilot study results.[36] Therefore, the present study will include a total of 32 participants with 16 participants per group. In addition, the results of this pilot study will provide information on the difference between groups and SD necessary for sample size calculation during the future full-scale study.