  Personalization is very powerful in improving the effectiveness of health
interventions. Reinforcement learning (RL) algorithms are suitable for learning
these tailored interventions from sequential data collected about individuals.
However, learning can be very fragile. The time to learn intervention policies
is limited as disengagement from the user can occur quickly. Also, in e-Health
intervention timing can be crucial before the optimal window passes. We present
an approach that learns tailored personalization policies for groups of users
by combining RL and clustering. The benefits are two-fold: speeding up the
learning to prevent disengagement while maintaining a high level of
personalization. Our clustering approach utilizes dynamic time warping to
compare user trajectories consisting of states and rewards. We apply online and
batch RL to learn policies over clusters of individuals and introduce our
self-developed and publicly available simulator for e-Health interventions to
evaluate our approach. We compare our methods with an e-Health intervention
benchmark. We demonstrate that batch learning outperforms online learning for
our setting. Furthermore, our proposed clustering approach for RL finds
near-optimal clusterings which lead to significantly better policies in terms
of cumulative reward compared to learning a policy per individual or learning
one non-personalized policy across all individuals. Our findings also indicate
that the learned policies accurately learn to send interventions at the right
moments and that the users workout more and at the right times of the day.
