## 5 Data Analysis
### Mean and Maximum Biofeedback Differences for Each Session  
The subjects' biofeedback data including skin conductance, body temperature, respiration, and heart rate were collected while performing the experiment in each session to assess changes over the VRET sessions. Base heart rate and base respiration rate differed between individuals, reducing reliability to compare mean biofeedback values between different sessions. The data were standardized before comparison. Comparisons of the maximum and mean value difference biofeedback were used to standardize the heart rate and respiration rate. The magnitude of the subjects' heart rate and respiration rate remained stable between different experimental scenarios and affected the intensity of the stimulus. For driving anxiety, emotion plays an important role in maximum and mean value differences for biofeedback. Larger value differences indicate a stronger anxiety response.  
While performing data analysis, a 99 percentile data level was used to collect each session's maximum biofeedback value to avoid outliers and changes within VRET sessions for the four driving scenarios of biofeedback (Figure 9). The value of mean and maxi  
Figure 8: (**a**) One-way ANOVA interval plot from anxious driving behavior questionnaire (Q1–Q19); (**b**) one-way ANOVA interval plot from anxiety-related-avoidance questionnaire (Q20–Q32); and (**c**) one-way ANOVA interval describing specific driving environment questionnaire responses (Q33–Q39).  
mum biofeedback differences progressively increased over sessions, with the exceptions occurring in skin conductance differences observed in level 4 and respiration differences observed in level 3. As a whole, the subjects' anxiety arousal became stronger over sessions, indicating subjects were more anxious driving across long bridges and on mountain roads.  
In our previous VRET experiment, we conducted a pre-test experiment with 31 subjects using a seven-level exposure therapy design (referring to [11]). Some systemic problems were discovered during the experimental process since the environment was not realistic, which lessened the VR immersive experience. These problems were improved in this experiment with the programming of a more realistic environment, closer to actually driving a car. Comparisons of maximum and mean value differences for biofeedback data between the previous experiment and the current experiment were conducted. The subjects' biofeedback shows significant differences between the two experiments. The results are presented in Figure 10; the blue line represents the current experiment (4-level design) and green line represents previous experiment (7-level design).  
Although we were unable to perform a pairwise comparison with our previous and current experiment, we observed an improvement with the enhanced immersive environment in the second experiment. The mean and maximum biofeedback differences collected from the current experiment are higher than the differences collected from the first experiment. The data show that subjects participating in the current experiment received increased intensity from a more realistic driving anxiety scenario. Although we have no evidence that VRET with a stronger anxiety-provoking scenario design will enhance the effectiveness of the treatment, the improved design with increased visual immersion induced a higher feeling of presence and improved driving performance on the part of the subject. VRET environments that increase the subjects' anxiety are believed to be more effective for driving training and exposure therapy [23].  
Figure 9: (**a**) Mean and maximum skin conductance differences; (**b**) mean and maximum body temperature differences; (**c**) mean and maximum respiration differences; and (**d**) mean and maximum heart rate differences.