## 5 Conclusions  
This research investigates the potential of rapid prototypes in immersive virtual environments (IVEs) for user testing. Such virtual environments embody novel ways of design and testing interactions with products in the early stages of development. However, these innovative research and development approaches are still poorly understood. Hence, this paper studies which characteristics of VR testing are beneficial to the NPD process and provides a better understanding of the shortcomings. The main conclusions of this work were, first, that when it comes to the main attributes of user testing and rapid prototyping in VR, the visual fidelity (being able to holistically grasp the product and the environment in a visual way) is the main advantage VR testing delivered to enhance the design phase within a multistakeholder NPD process. More specifically, VR enabled a level of visualization beyond the traditional capabilities of 3D renders or sketches. As a consequence, during the early and middle stages of the development process, this establishes better mutual conceptual understanding between the wide variety of (internal and external) stakeholders. In future research, this could be extended to mixed reality settings, which would allow for more realistic visual elements (see, for example, [PERSON] and [PERSON] [38]), e.g., testing with full-colour passthrough or lidar environment scanning on newer types of hardware combined with multi-person testing. This could be an interesting follow-up study to explore more contextual realism in the visual domain. Second, besides the visual fidelity, this study also revealed that VR testing is perceived as more interactive and realistic compared to paper prototyping, which could be labelled as the degree of interaction fidelity in line with the research of [PERSON] [11]. As innovative products are increasingly interactive in nature, entailing complex, ubiquitous, embodied, contextual interactions, such interaction fidelity will gain importance and traditional screen-based prototyping and testing will be challenged and the interactions will become harder to design and research. However, the potential of VR testing is dependent on a couple of boundary conditions that need to be well-understood and taken into account to maximize the fidelity of the test. The first boundary condition is the realism in VR environments compared to the real world. Although the interaction fidelity of VR environments is higher compared to paper prototyping, and users are able to understand and experience the product at a conceptual level, some more complex interactions are still perceived as rather difficult and can impact the usability assessment of the system. As discussed by [PERSON] et al. (2019), for example, MR applications could relate to more realistic test user behaviour compared to VR applications. Similarly, using hand tracking in combination with haptic interfaces interactions could further improve the interaction fidelity (as proposed by [PERSON] et al. (2019)). A second boundary condition is the relationship between the medium and the message. Being in VR is still new for a lot of people, which causes interrelated behaviour (being in the medium and interacting with the medium), e.g., when onboarding and learning to work with the technology. One should consider their degree of familiarity with VR and consider sufficient warming-up, explanation and time to become familiar with the system (decreasing medium-related novelty effects, joy bias and errors). A final boundary condition is the social context. While the interactions and physical environment are adequately experienced, the validity of multi-user/multi-agent interaction contexts require high levels of agent realism. Merely adding static, non-responsive avatars is insufficient to fully represent the intricacies of such complex contexts. Nevertheless, this has a high impact on the way the product is being used and experienced.