## 5 Discussion  
The observations of the participants during the experiment and their comments during the informal interview at the end of the experiment provided interesting information to interpret the results. The initial reaction of participants when they tried a VR HMD for the first time was astonishment and enthusiasm. All of them seemed much more excited to work in VR than with the desktop program. The 20 min they were allowed to practice freely were short when they did this with EVM, and they wanted to continue modeling and experimenting. However, with SketchUp, the 20 min were very long, and they wanted to move on to the next experimental phase sooner. This is a relevant fact to consider EVM as a potential learning tool in an academic setting, as learner motivation is an important part of any learning experience. If learners are not motivated, no matter how effective a particular tool is, they will not use it. This observation confirms the results of previous works [46; 47; 48] and reflects the positive impact of VR in increasing student motivation.  
Regarding the execution time of the modeling tasks, the observation of the behavior of the participants may explain why SketchUp outperformed EVM in four of the six modeling tasks. We highlight two key aspects:  
1. When modeling in the VR-based environment, participants tended to create models that were much larger in size than those created in SketchUp, which forced users to move around the virtual environment and perform actions that required the movement of their arms. Working with SketchUp, however, only requires subtle movements of the hands and wrists, which undoubtedly can be performed much more quickly. This behavior is similar to other studies [23] that reported physical fatigue in the VR condition due to the physical activity required to sketch and model in that context;
2. When modeling in the VR system, participants tended to spend a significant amount of time looking at the model and the environment. This behavior is common in users who have never experienced an immersive virtual reality environment before. This behavior perhaps is related to observations in similar experiments [24] that found a simpler mental image manipulation as one of the positive characteristics of VR sketching. The participant does need to create a 3D mental representation before sketching, as a virtual 3D representation supports cognitive processes.  
Figure 12: Relative SUS and ease of use scores per question.  
It is important to mention the important difference in the perceived usability measured by the SUS score. Considering the extensive analysis performed in [49], the high score (92.3) obtained by EVM is very relevant and can be rated as excellent. The SketchUp score was modest (74.3), which can be rated as good. These results can be partially explained by considering the typical \"wow effect\" [50] that first-time VR users experience.  
Regarding the ease of use, the similar scores of EVM and SketchUp and the fact that no statistically significant differences were found in the questionnaire's overall score may be explained by the fact that the ray-casting and panel menu combination used in EVM are conceptually very near the typical WIMP interface [51], which is well known by participants. Mouse cursor operation and selection of menu options by clicking the mouse on an icon are replaced by moving the controller in space and using ray-casting to select the desired icon.