## 4 Discussion  
This study conducted an experiment for finding out the stress differences between experienced experts and students in SA in maritime navigation. The result of data analysis shows that the biosignal data from experts and students can be classified by a certain machine learning algorithm. The result of subjective measurement of workload shows that there is a difference between experts and students. The summary of findings is shown in Table 5.  
\begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**Feature Types** & \begin{tabular}{l} **Validation** \\ **Error** \\ \end{tabular} & \begin{tabular}{l} **testError** \\ \end{tabular} & \begin{tabular}{l} **testError95 CI** \\ \end{tabular} & \begin{tabular}{l} **Accuracy** \\ \end{tabular} \\ \hline \begin{tabular}{l} Statistic-base + wavelet-based \\ features (method 1) \\ \end{tabular} & 0.357 & 0.357 & 0.193 & 0.522 & 64.3\% \\ \hline \
\begin{tabular}{l} HOC-base features (method 2) \\ \end{tabular} & 0.263 & 0.263 & 0.119 & 0.407 & 73.7\% \\ \hline
\begin{tabular}{l} Statistic-based + \\ Wavelet-based + HOC-based \\ features (method 3) \\ \end{tabular} & 0.245 & 0.245 & 0.101 & 0.388 & 75.5\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Bayesian Optimization Results for CNN from different feature selections.  
Figure 5: Histogram of Bayesian optimization results for CNN from different feature selections. In this figure, method 1 represents the selection of statistic-based feature and wavelet-based features, method 2 represents the selection of HOC-based features, and method 3 represents the selection of statistic-based feature, wavelet-based features and HOC-based features.  
Based on the results associated with the three hypotheses, we outline the following statements:  
**Statement 1.** Biosignal data are considered as stress monitoring analysis, since stress can be a physical, mental or emotional reaction, and it causes hormonal, respiratory, cardiovascular and nervous system changes. For example, stress can make your heart beat faster, make you breathe rapidly, sweat and tense up. Based on the results of the classification of the biosensor data, we can see that the data from the experts and the students have different patterns. Accuracy of 75.5% is an acceptable result for distinguishing the data. This could have implication for stress difference with maritime navigating skills. Previous research shows that experts obtain better results in SA task [38]. When facing the same task, with different level of skills, the stress level is different.  
**Statement 2.** NASA-TLX contains subjective data which was evaluated by the participants themselves. Results from the NASA-TLX show that experts and students had different evaluation of the workload, which is consistent with the results from classification of biosignal data, i.e., biosignal data show a different pattern for experts and students. This result is also consistent with other research suggesting that mental effort and anxiety are closely related to HRV [39].  
**Statement 3.** The results of NASA-TLX rating show that experts have a smaller workload compared to students. Many research articles show that there is a high correlation between workload and stress. When there is the overload of the work, the stress is increasing [40; 41].Therefore, there have implication that experts feel less stress than students. In addition, some other research also shows that higher levels of stress had a negative relationship with work SA [42]. Since experts have better SA score [38], they should be under less stress.