Impact of a Mobile App-Based Waitlist Intervention on Readiness for Eating Disorder Treatment

Change from Baseline in Overall Motivation for Treatment as Measured by the Readiness and Motivation Questionnaire (RMQ) at 4 Weeks

Overall motivation for treatment will be assessed using the Readiness and Motivation Questionnaire (RMQ), a validated measure evaluating readiness to change eating disorder symptoms and behaviors. The RMQ provides a total motivation score (ranging from 0 to 100) and includes subscales for precontemplation, action, internality, and confidence. Higher scores indicate greater readiness for treatment. Changes from baseline to 4 weeks will be evaluated using linear mixed modeling (LMM), accounting for fixed effects (treatment group, time) and random effects (individual variability). A significant change from baseline will be defined as a statistically significant fixed effect of time (p \< 0.05), with a greater increase in motivation scores observed in the treatment group compared to the delayed treatment control group.

Rate of Change in Weekly Motivation Levels as Measured by the Motivational Stages of Change for Adolescents Recovering from an Eating Disorder (MSCARED) During the Initial 4 Weeks

Weekly motivation will be assessed using the Motivational Stages of Change for Adolescents Recovering from an Eating Disorder (MSCARED), a single-item self-report measure. Participants select their current stage of change from six stages: precontemplation, contemplation, preparation, action, maintenance, and recovery. Rate of change will be calculated using LMM, incorporating interaction terms between treatment group and time to evaluate differences in the trajectory of weekly motivation. A significant result will be defined as a statistically significant interaction term (p \< 0.05), indicating a slower decline or greater increase in motivation over time in the treatment group compared to the delayed treatment control group.

Change from Baseline in Confidence to Engage in Treatment as Measured by the Confidence Subscale of the Readiness and Motivation Questionnaire (RMQ) at 4 Weeks

Confidence to engage in treatment will be assessed using the confidence subscale of the RMQ, which measures participants' belief in their ability to address eating disorder behaviors. Scores range from 0 to 100, with higher scores indicating greater confidence. Changes from baseline to 4 weeks will be analyzed using LMM, incorporating fixed effects for treatment group and time, and random effects for individual differences. A significant change from baseline will be defined as a statistically significant fixed effect of time (p \< 0.05), with confidence scores in the treatment group showing stability or improvement compared to the delayed treatment control group.

Attrition Rate Between Baseline and 4 Weeks

Attrition will be measured as the percentage of participants who do not complete study activities (e.g., assessments or app usage) within the first 4 weeks. Descriptive statistics, including means, percentages, and ranges, will be calculated to summarize attrition rates for both groups. Simple t-tests will be used to compare mean attrition rates between the treatment and delayed treatment control groups, with a significant difference defined as a p-value \< 0.05, indicating lower attrition in the treatment group.

Participant Ratings of App Acceptability as Measured by the User Version of the Mobile App Rating Scale (uMARS)

Acceptability of the MI-Coach: ED app will be assessed using the User Version of the Mobile App Rating Scale (uMARS). This scale includes subscales for engagement, functionality, aesthetics, and information quality, each rated on a 5-point Likert scale (1 = inadequate, 5 = excellent). Descriptive statistics (means, standard deviations, ranges) will summarize acceptability scores for each subscale and overall scores. Scores averaging ≥ 4 will indicate a "high" or "very high" level of acceptability for the app.

Association Between App Usability and Module Completion Rates

App usability will be assessed using the ease of use and perceived usefulness subscales of the uMARS. Engagement metrics, including the number of modules completed, days the app was used, assessments and exercises completed, and logins, will be obtained from in-app data provided by Resiliens Inc. Generalized linear regression models will evaluate the relationship between usability scores and app engagement metrics, with significant associations defined as a p-value \< 0.05. Descriptive statistics will summarize app engagement metrics.

Exploratory Analysis of Changes in Anxiety Symptoms as Measured by the Generalized Anxiety Disorder-7 (GAD-7) Scale

Anxiety symptoms will be assessed using the GAD-7, a validated self-report measure with scores ranging from 0 to 21 (higher scores indicate greater severity of anxiety symptoms). Descriptive statistics, including means and standard deviations, will summarize anxiety scores at baseline, 4 weeks, and follow-up time points. Exploratory analyses will use simple t-tests to compare changes in mean anxiety scores between the treatment and delayed treatment control groups at 4 weeks. Changes over time will also be analyzed using linear mixed modeling (LMM) to evaluate group differences in trajectories. Statistical significance will be defined as p \< 0.05 for group comparisons or time effects.

Exploratory Analysis of Changes in Body Dissatisfaction as Measured by the Body Shape Questionnaire (BSQ)

Body dissatisfaction will be assessed using the BSQ, a 34-item self-report measure with scores ranging from 34 to 204 (higher scores indicate greater dissatisfaction with body shape). Descriptive statistics will summarize scores at baseline, 4 weeks, and follow-up time points. Simple t-tests will compare mean changes in body dissatisfaction scores between the treatment and delayed treatment control groups at 4 weeks. LMM will explore whether group differences emerge in body dissatisfaction trajectories across time points, with statistical significance defined as p \< 0.05.

Exploratory Analysis of App Engagement Metrics and Clinical Outcomes

App engagement metrics, including the number of modules completed, days the app was used, assessments and exercises completed, and logins, will be extracted from in-app data provided by Resiliens Inc. Exploratory analyses will examine associations between engagement metrics and changes in clinical outcomes (e.g., motivation, anxiety, body dissatisfaction). Generalized linear regression models will evaluate the predictive relationship between engagement metrics (independent variables) and changes in outcome measures (dependent variables). Descriptive statistics will summarize engagement patterns, and statistical significance will be defined as p \< 0.05 for predictive associations.

Exploratory Analysis of Changes in Eating Disorder Symptomatology Over Time

Eating disorder symptomatology will be assessed using the RMQ symptom-specific subscales (e.g., cognitive, bingeing, compensatory strategies, restriction), with scores ranging from 0 to 100. Exploratory analyses will examine changes in symptom-specific subscale scores between the treatment and delayed treatment control groups using t-tests for between-group differences at 4 weeks and LMM to evaluate changes over time. Descriptive statistics will summarize baseline and follow-up scores, with statistical significance defined as p \< 0.05 for group differences or time effects.