The LvL UP Pilot Trial: Assessing the Feasibility of a Sequential, Multiple Assignment, Randomized Controlled Trial to Evaluate the Effectiveness of a Blended Mobile Lifestyle Intervention

Participants will:

• Visit the study site at baseline and week 8 to complete assessments.
• Complete an online assessment at week 4.
• Be assigned to group 1 (intervention) or group 2 (comparison).
• Participants in group 1 will download the LvL UP App.
• Participants in group 1 categorized as 'non-responders' at week 4 will receive four motivational interviewing coaching sessions in addition to continue using the LvL UP app (LvL UP + MI).
• Participants in group 2 will receive a package with standard lifestyle resources from Singapore's Health Promotion Board.

Participants will be randomly allocated to groups 1 or 2 (LvL UP or comparison) following a 2:1 ratio favoring the LvL UP group. At week 4 (decision point), participants from the LvL UP group will be classified as responders or non-responders based on pre-specified criteria. Non-responder participants will be re-randomized with equal probability (1:1) to one of the two second-line conditions: (i) continuing with the initial intervention (LvL UP) or (ii) additional MI support sessions (LvL UP + MI). Response / non-response categorization will involve assessing preliminary intervention effects (positive change in one of the following: physical activity, diet or mental well-being) as well as satisfaction with the intervention (net promoter score).

Participants will also take part in a process evaluation informed by the UK Medical Research Council's guidelines (Moore et al., 2015) to explore implementation (process, fidelity, dose, adaptations, reach), mechanisms of action (participant experience and response to intervention, mediators, unexpected pathways and consequences), and contextual factors that may affect implementation and intervention outcomes. Methods will entail qualitative and quantitative approaches, including surveys, interviews, web-based and app-based analytic data, and direct observation. In addition, other measures have been added to the research protocol to address the following exploratory aims: (i) explore time-varying and baseline moderators on intervention outcomes (e.g., sociodemographic variables, personality), (ii) identify the most cost-effective intervention condition from the societal perspective (e.g., self-reported sickness absence), and (iii) investigate which LvL UP app and smartphone sensor data (e.g., usage patterns, location) might be effective in predicting participant's engagement, state of receptivity to LvL UP notifications, or state of vulnerability (mental health conditions). All measures are described in the outcomes section.

To formalize the sample size calculations for the pilot study, the precision-based approach by Yan et al. (2020) was used to ensure the estimated outcomes for LvL UP and LvL UP + MI are controlled within a certain precision (i.e., the margin of error as a proportion of the outcome's standard deviation). Assuming a 50% responder rate, a two-tailed Type I error of 5% and a precision of 30%, a total of 97 participants will be required. Taking an 82% retention estimate for digital health interventions lasting ≤8 weeks (Jabir et al., 2023) and rounding up the number, 120 participants will be recruited. Therefore, 40 and 80 participants will be randomly assigned to the comparison condition (group 2) and LvL UP (group 1), respectively. In addition, an estimated 40 participants from group intervention will be re-randomized to continue with LvL UP or have LvL UP + MI. Considering participants assigned to group 1 are asked to nominate a LvL UP Buddy, which will be part of the process evaluation of the trial and thus technically considered participants, the total estimated sample size is 200 (120 trial participants and 60 LvL UP Buddies).