## 1 Introduction
### Related Work  
Collaborative learning may be defined as a coordinated, synchronous activity, which results from a continued attempt to construct and maintain a shared understanding of a problem [14]. As elaborated by [15], collaborative learning can be analysed from a variety of perspectives, such as types of interactions, characteristics of a situation, mechanisms for learning, and effects of collaboration. Some of the key mechanisms enabling collaborative learning include (i) equality of interaction partners, who have a common goal and can perform the same actions while working together; (ii) interactivity of peers, who influence the cognitive processes of interaction partners; (iii) synchronicity of communication; (iv) sharing work in the group rather than splitting work and working on sub-tasks individually; and (v) possibility to negotiate, argue, and justify standpoints [15]. From this perspective, the game-based activity of planning poker can be classified as a collaborative learning activity, in which sharing a common goal, working on the same tasks, engaging in synchronous communication, negotiating, justifying, and influencing one another's standpoints with the aim of reaching a consensus are the core of the game play. In general, collaboration has been considered beneficial for learning since it allows bringing together different perspectives, complementary abilities and skills to joint activities aimed at reaching a shared goal and/or solving a shared problem [16; 17]. Peer interaction is an important element of collaboration as it provides opportunities for learners to support one another through information sharing and communication [15; 16; 17]. Moreover, collaborative learning activities may have a positive effect on the sense of self-efficacy and self-regulation [15; 16]. Collaboration in the sense of integration and synthesis of ideas and expertise of all group members [16] is a crucial part of planning poker in which group members share perspectives and discuss diverse standpoints in order to reach a consensus. Robot-Supported Collaborative Learning (RSCL) was proposed as a term by [8] to describe an approach in which a robot supports a collaborative learning activity. Compared with technologies used in Computer-Supported Collaborative Learning (CSCL), social robots are applied not only to mediate interactions but also to act as interaction partners [18]. In RSCL, the teacher and the robot have a shared presence in the classroom [18]. From this perspective, RSCL differs from CSCL in that a social robot is used as a co-present, embodied technology with a social role [18]. Furthermore, research studies, such as [19], showed that robot-supported social interaction in a small group enhances the quality of social presence. Applying a social robot as a facilitator of collaborative learning in small groups can be also helpful for enhancing learners' attention and motivation [20]. A wide range of approaches used in CSCL to increase the probability of collaboration, such as carefully designing interactions, roles, rules, methods of scaffolding, and monitoring [15], can be applied and/or adapted to designing RSCL. While applying social robots to support collaborative learning in small groups in higher education settings is still a novel approach, previous studies have explored some important aspects and highlighted the potential of applying social robots such as NAO to facilitate collaborative activities in small groups of students. For example, the study by [8] reported on learning activities in small groups facilitated by the NAO in an undergraduate university course on human-computer interaction. The goal of the study was to compare three conditions of instruction: a human facilitator, a robot facilitator, and facilitation with tablets. In this study, groups of four students performed a group activity, followed the instructions of a facilitator, discussed the material, and answered questions about the interaction. The study revealed a number of benefits and drawbacks of using a robotic facilitator.