**2. System Description**

To the user, the Active2Gether system presents itself as an Android-based mobile phone app that continuously monitors the context of a person. One of the distinct features of the system is that it implements evidence-based behavior change techniques, unlike most apps that are currently available in the app stores [9]. The most promising behavior change techniques are employed in the app, including self-monitoring, performance feedback, goal setting and social comparison.

The app performs four main functions: it communicates with the user about his/her objectives regarding physical activity for the next week, provides timely and personalized feedback, facilitates self-monitoring based on several collected data sources, and supports social comparison with the help of Facebook friendship relations. The system focuses on three types of physical activity: leisure time sports activities, active transport and stair walking. Users can choose to be coached on at most one of these three domains at the same time.

In the following sections, we describe the design of the system in detail. We first describe the data that are collected by the system. We then provide an overview of the architecture of the system and describe the layout choices. After that, we explain the working of the reasoning engine and the selection and filtering of coaching messages. Finally, we describe the implementation of the social comparison functionality.
