*2.5. Selection and Tailoring of Coaching Messages*

Once the order of the most promising coaching strategies is established (as described in Section 2.4.3), all necessary elements to send the coaching messages are in place. As explained before, each coaching strategy represents a set of messages targeting one of the concepts from the computational model. The messages are based on established behavior change techniques (e.g., prompting specific goal setting, time management; see [22] for a complete taxonomy), while also taking into account user preferences [23]. They were written to be motivational, personally relevant and trustworthy, and were annotated with restrictions for the circumstances under which they are relevant (e.g., day and time, the user's awareness phase and coaching domain, the user's perceptions reported in the intake questionnaire). A detailed description of the techniques that are applied in the messages and the related determinants is provided in [7]. The messages are sent up to three times a day. In order to send a message, it has to be selected from the set of available messages and (if applicable) tailored to the user.

At given moments in time, the communication engine of the system checks for messages that are relevant to send to the user. Selecting a message is based on elimination: starting from the set of all messages, the selection is narrowed down by filtering inapt or irrelevant messages. As explained in Section 2.4, the user is already assigned an awareness phase and has chosen a coaching domain. In addition, a coaching determinant is picked from the ordered list, with a probability relative to its position in the list. This probability is introduced to increase diversity in the messages during the week. Then, all messages that are aimed at other awareness phases, coaching domains or targeted coaching determinants are filtered out. The system checks for other aspects of the message's relevance as well, such as the day and time, the user's occupational status, answers to questions in the intake questionnaire, whether the user is on track to reach their goal and the current weather.

Once a message is selected, if there are open fields, they have to be filled in to tailor the message to the user and context. In order to increase relatedness, most messages address the user with their first name. Additionally, some messages are completed by filling out the user's current daily number of steps or stairs, their accumulated weekly time spent on sports or active transport, their weekly goal, the percentage of their weekly goal they have reached so far, the maximum number of stairs they would consider to walk, or the current weather score.

The coaching strategies are only relevant if the user is in the coaching phase (see Section 2.4), as only then certain determinants from the computational model are targeted. However, also when users are in the education phase or in the feedback phase, they receive messages on a regular basis. In the education phase, users receive messages that put their (insufficient) performance into perspective, as well as messages that emphasize the need for and benefits of physical activity. In the feedback phase, users receive positive feedback on their satisfactory behavior. In addition, users in all awareness

phases may receive general messages, regarding their current daily step or stairs count, containing universal motivational phrases, or to remind them to charge their Fitbit or to synchronize their data.
