Murphy, Susan A.Ghosh, Susobhan2026-06-0920262026-05-122026Ghosh, Susobhan. 2026. Deployable Online Reinforcement Learning Algorithms for Use-inspired Research in Digital Health. Doctoral Dissertation, Harvard University Graduate School of Arts and Sciences.32701622https://dash.harvard.edu/handle/1/42740465Online Reinforcement Learning (RL) algorithms have emerged as a promising approach for personalizing just-in-time adaptive interventions (JITAIs) in digital health. Yet, their real-world deployment presents challenges that extend well beyond policy optimization. In digital health settings, online RL algorithms must learn under severe data scarcity and noise, operate autonomously within clinical constraints, remain interpretable and reproducible, and support valid post-deployment scientific analysis. This thesis focuses on methodologies to overcome these challenges to successfully design and deploy online RL in digital health. To support the continual improvement process of iteratively designing, testing, refining, and re-deploying algorithms, this work first details a reproducible workflow for online AI in digital health, structured across algorithm design, system assurance, and post-deployment analysis. Building on this foundational workflow, the thesis details practical algorithmic solutions to the aforementioned challenges of online RL in digital health. Addressing the algorithm design phase, the thesis presents reBandit, a random effects-based online RL algorithm developed and piloted in the MiWaves digital intervention to reduce cannabis use among emerging adults. To overcome severe data constraints, reBandit leverages informative Bayesian priors and random effects to adaptively pool data across the user population, enabling efficient learning while also personalizing to the individual. To enhance the stability and autonomy of a deployed algorithm, the work outlines a framework for designing algorithm monitoring systems. For post-deployment analyses, the thesis introduces a resampling-based method to evaluate whether a deployed online RL algorithm successfully learned to personalize treatments. Additionally, it investigates the user experience of participants in the MiWaves pilot study, presenting insights on self-awareness, burden, and intervention personalization to inform the next iteration of the intervention and algorithm. Finally, this thesis concludes by summarizing its key contributions and outlining promising directions for future research.application/pdfenDigital HealthMobile HealthMulti-armed banditsOnline RLReinforcement LearningComputer scienceThesis or Dissertation2026-06-090000-0003-3654-4141