## 1 Introduction
### Evaluating VR Prototyping  
In this study, we compare VR to non-VR prototyping in different ways, firstly focusing on experimental control and external validity in order to receive valuable insights into the user experience. The testing of contextual realism as a metric of external validity was viewed through the context framework of [PERSON] [37]. In their research, external validity is defined as whether an observed causal relationship should be generalized to and across different measures, persons, settings and times. For this purpose, the field test and context in VR were introduced. In his paper, [PERSON] [38] defines realism and control as two major trade-offs when testing in situ or in vitro. Related to this, [PERSON] [39] uses a graph for mapping research methods in respect to these two major trade-offs (see Figure 1). The blue rectangles indicate the hypothesised mapping of the research techniques discussed in this study: testing in the field (TIF) and virtual reality testing (VRT). This study envisions VRT to cause more experimental control compared to TIF. Also, VRT should allow similar levels of ecological validity compared to TIF. Next, to control the validity, other important aspects of VR are the presence and realism of VR experiences. The optimisation of these aspects sees continuous improvement in correspondence with the development of VR technology throughout the years. For one, the visual fidelity of headsets has been improving and the production cost of lenses implemented in these headsets is becoming cheaper. Also related to the hardware development of VR technology, immersive interaction (hand tracking) and motion tracking (full body tracking) are becoming more available. This results in more accurate depictions of virtual  
Figure 1: [PERSON] landscape of user research methods [39]. avatars and richer interaction possibilities. Together with the improved lenses, higher rendering quality and more immersive interaction/tracking, the level of realism keeps improving. This study borrows its definition of realism from [PERSON] [40]. According to her, realism can be broken down into four relevant features: scene realism, audience behaviour, audience appearance and sound realism. This research mainly targets scene realism. The perceived realism can be measured using the six-item questionnaire developed by [PERSON] [40]. These questions were adapted for the depiction of realism in films to virtual reality. The participants indicated the degree to which they perceived each of the six elements using a response scale that varied from 0 (no, not at all) to 10 (yes, entirely). In accordance with the definition of scene realism by [PERSON] [40], this study hypothesises that the VR prototype is more realistic compared to the low-fidelity prototype tested in the field. Closely related to realism is the level of presence a user experiences while being in VR. A modern definition is proposed by [PERSON] [41]: \"Presence within the context of virtual reality is defined as one's sense of being in the virtual world. The illusion is perceptual but not cognitive, as the perceptual system identifies the events and objects and the brain-body system automatically reacts to the changes in the environment, while cognitive system slowly responds with a conclusion of what the person experiences is an illusion\". [PERSON] [42] developed a presence scale for use in VR applications. In line with this, we hypothesised that the participants felt more present when using the VR prototype compared to the low-fidelity prototype. Interactivity means, according to [PERSON] [43], \"The degree to which users of a medium can influence the form or content of the mediated environment.\" To facilitate interaction, interactive prototypes must be built. Before VR prototyping, prototypes were mainly electronic or screen-based prototypes. These fulfil the need for a functional prototype. This means providing the visual appearance in combination with the intended design.