2) 20 to 39 years old
3) Beck Depression Inventory-II (BDI-II) score of 10 points or more
4) Written informed consent provided prior to participation.
1) Being diagnosed with a psychiatric disorder at least once in their lifetime, regardless of its type.
2) Currently receiving treatment from a professional for mental health issues.
3) Vision or hearing problems that interfere with daily life.
4) Having had a major depressive episode in the last two weeks according to the Mini-International Neuropsychiatric Interview (MINI).
Generally, the cutoff value for BDI-II is 14 points or higher. However, several previous studies on StD have used BDI-II with a cutoff value of 10 points or higher. Therefore, in this study, the cutoff value for BDI-II was set at 10 points or higher.
The mean (SD) age of participants who met the eligibility criteria was 20.88 (0.72) years for the experimental group and 21.19 (1.68) years for the control group. The percentage of women was 100% in the experimental group and 87.5% in the control group. The percentage of alcohol consumption was 25% in the experimental group and 6.3% in the control group. The mean (SD) duration of sleep was 6.22 (1.28) hours for the experimental group and 6.97 (1.06) hours for the control group. The percentage of smokers was 0% in both the experimental and control groups. The mean (SD) score of the BDI-II was 17.00 (7.03) for the experimental group and 15.25 (3.28) for the control group (Table 1). 
Participants were recruited from Gunma University in the Gunma Prefecture, Japan, from March to June 2021. Advertising, e-mail, and social networking services were used to recruit participants. Potential participants who wished to participate in the study were given a detailed explanation of the purpose and procedures of the study using handouts. The URL of the online questionnaire was then sent to potential participants interested in the study to obtain electronic informed consent. Consent to participate in the study was obtained by the participant selecting the consent option. Participants who gave informed consent continued to complete the online questionnaire to assess their eligibility. The researcher then interviewed potential participants using the MINI. After completing a baseline assessment, participants were randomly assigned to the experimental or control groups. The baseline assessment was conducted by the investigator within one week after the eligibility assessment was conducted.
The experimental group received a video viewing intervention using the SPSRS application, while the control group received a video viewing intervention via YouTube.
The experimental group received a video viewing intervention using the SPSRS smartphone application. Two types of word stimuli are displayed in the SPSRS application. First, "able", "let us try", "good luck", "can", and "do not worry" were randomly displayed in the four corners of the screen as word stimuli to boost confidence. These words were displayed for 17 milliseconds. Immediately after that, positive word stimuli, "great", "fantastic", "nice", "satisfactory", and "enjoyable", are displayed in the center of the screen for 150 milliseconds. These words appeared on the screen repeatedly every five seconds. The SPSRS application was installed on iPhones managed by the research team, and participants watched a predetermined 10-minute video (of a basketball game). Originally, the SPSRS application was designed for participants to independently select and watch videos. However, we decided to select the videos to be viewed during the design phase of the study because the influences of different videos on depressive mood may vary across participants. It is unclear whether the videos we selected are optimal for this study. However, some studies suggest that watching sports may improve viewers' moods. In addition, the SPSRS application has a filtering function that prevents participants from viewing videos that may be inappropriate for them (eg, violent videos). Therefore, the videos of basketball players used in this study were unlikely to disturb participants and were considered appropriate. Intervention implementers used the SPSRS application following the user manual developed by the research team to ensure fidelity of the intervention.
The control group watched the same video as the experimental group for 10 minutes using the YouTube application. Therefore, there was no confidence-boosting or positive words in the video. The control group used the same iPhones as the experimental group and also received the intervention by the same provider as that of the experimental group. As in the experimental group, the intervention implementers used the YouTube application following the user manual developed by the research team to ensure the fidelity of the intervention.
The study collected demographic data such as age and gender as well as lifestyle characteristics such as sleep duration, smoking habits, exercise habits, and drinking habits as part of the baseline assessment. Sleep time was defined as the average sleep time per week. Drinking habits were defined as drinking at least one bottle (approximately 500 mL) of beer per day, >=3 days a week. Smoking habits were defined as "daily smoker" or "have occasional smoking days." Exercise habits were defined as exercise periods "once 30 min or more" performed at least twice a week and continued for at least 1 year.
The primary outcome of the study was the change from baseline in depression-dejection on the Profile of Mood States 2nd Edition-Adult Short (POMS 2-A Short) after the intervention. The POMS 2-A Short is a 35-item self-administered questionnaire for measuring mood state. It consists of seven subscale scores (anger-hostility, confusion-bewilderment, depression-dejection, fatigue-inertia, tension-anxiety, vigor-activity, and friendliness). Each subscale is rated on a 5-point Likert scale (0 = not at all, 1 = a little, 2 = moderately, 3 = quite a lot, and 4 = extremely). High scores indicate better vigor-activity and friendliness are better but more severe symptoms in the five other domains. In addition, the Total Mood Disturbance score (TMD) is calculated from anger-hostility, confusion-bewilderment, depression-dejection, fatigue-inertia, tension-anxiety, and vigor-activity. TMD is calculated based on a standard value (mean of 50 and standard deviation of 10). POMS 2-A Short showed high internal consistency (Cronbach's alpha = 0.80-0.95).
Anger-hostility, confusion-bewilderment, fatigue-inertia, tension-anxiety, vigor-activity, friendliness, and TMD as measured by the POMS 2-A Short.
The STAI-S is a 20-item self-administered questionnaire used to measure anxiety state. Each item on the STAI-S is rated on a scale of 1 to 4, with a total score of 20 to 80. The higher the score, the stronger the anxiety state. STAI-S showed high internal consistency (Cronbach's alpha = 0.87-0.91).
This was an open-label study in which participants, interventionists, and the assessor were not blinded. However, we tried to minimize the bias caused by a lack of blinding. First, a baseline assessment was performed prior to the random assignment. Second, we also used standardized methods to measure outcomes for auditing and maintaining the quality of the data. In addition, the interventions for the experimental and control groups were conducted according to the manual developed by the research team.
Sample size calculation requires four factors: significance level (alpha = 0.05), power (80%), and group difference and standard deviation in primary outcome. However, group difference and standard deviation are determined based on past research data, and there is no research to date examining the immediate effects of the SPSRS application intervention on depressed mood in people with StD. Therefore, it was difficult to calculate the sample size based on actual data. In the absence of data from previous studies, a pilot RCT is a reliable way to obtain information prerequisite to sample size calculations (mean and standard deviation of differences between groups for the primary outcome) for later full-scale RCTs. In fact, one of the main objectives of this pilot study is to inform sample size calculations for subsequent full-scale RCT. For these reasons, pilot RCTs do not require formal sample size calculations. However, a sufficient sample size is necessary to ensure the scientific validity of the pilot study. In general, the sample size of a pilot study intended for subsequent full-scale studies requires 15 to 20 participants per group. Therefore, the goal of this study was to gather 16 participants per group, for a total of 32 participants.
Participants who met eligibility criteria were randomly assigned to either the experimental or control group in a 1:1 ratio after baseline assessment. The randomization list was created by a third party not involved in the study, using Excel software, and based on a permuted block method (block size 4). The prepared randomized list was sent to the central registration center established at the Kurashiki Heisei Hospital in Japan. Each time a participant was enrolled in the study, the central registration center was contacted and a random assignment was made. Thus, the process of randomization was concealed.
All randomly assigned participants were analyzed based on intention-to-treat (ITT). To investigate the effect of the SPSRS application on primary and secondary outcomes, a linear mixed model (LMM) with a restricted maximum likelihood estimation method was used. LMMs can use all available data to produce estimates. Therefore, LMMs can address the deficiencies that occur in clinical trials. The fixed-effect factors considered the group, time, and group and time interactions, while the random-effect factors considered the participants. The model used a type III test of fixed effects. SPSS v.27.0 (IBM Japan, Tokyo, Japan) was used for these analyses, with statistical significance set at P < 0.05 for the two-tailed test. We also reported the Hedges' g between groups to report the standardized mean difference. Hedges's g is capable of correcting the upward bias of Cohen's d caused by the size of the sample size. Hedges's g between groups was calculated pre and post, and Hedges's g adjusted for baseline differences was reported by subtracting the pre-Hedges's g from the post-Hedges's g. For the standardized mean difference between groups, 0.2 was considered a small difference, 0.5 a medium difference, and 0.8 a large difference.
It was approved by the Ethical Review Board for Medical Research Involving Human Subjects of Gunma University (approval number: HS2020-157) and was registered with ClinicalTrials.gov (NCT04707495). Each participant's data was recorded and processed as a participant number to protect the participant's privacy and maintain confidentiality. In addition, no information regarding the identity of the individual was entered into the electronic database. Names and other identifying information, such as informed consent, were stored separately from the survey materials identified by participant number.