## 3 Results
### Subjective Workload Measures in Different Automation Levels and Statuses  
Table 2 summarizes the descriptive statistics for overall subjective workloads by automation statuses and levels. Generally, a comparison of the effects of automation levels and status on the workload showed that as the automation level is increased, the overall subjective workload decreased in Routine automation (\(F\) (1, 89) = 4.57, \(p\) \(<\) 0.05) and increased in Failed automation (\(F\) (1, 89) = 5.61, \(p\) \(<\) 0.05). These patterns correspond to those in the lumber jack hypothesis of automation [3]. The participants perceived that the subjective workloads are sensitive to the automation levels and Failed automation imposed more workload than Routine automation. Specifically, Table 3 indicates that \"High and Failed\" automation shows the highest subjective workload in the four different automation settings, in which \"High and Routine\" automation can be considered as the most operable condition.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**NASA TLX Scale**} & \multicolumn{4}{c}{**Automation Level and Status**} \\ \cline{2-5}  & **Low and Routine** & **High and Routine** & **Low and Failed** & **High and Failed** \\ \hline Overall Subjective Workload & 36.1 (6.72) & 31.3 (5.58) & 66.0 (10.71) & 75.3 (8.01) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Overall subjective workload by automation level and status.  
Figure 4: Variations in longitudinal ST ratios in the four sessions.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Perceived Trust**} & \multirow{2}{*}{**Total N = 45**} & \multicolumn{4}{c}{**Subjective Workload Subscale (Mean)**} \\ \cline{2-5}  & & **Mental Demand** & **Effort** & **Perceived Performance** & **Temporal Demand** & **Frustration** \\ \hline Lower than expected & 7 & 13.8 A & 9.6 A & 13.4 A & 13.8 A & 14.0 A \\ \hline As expected & 11 & 8.6 B & 8.2 B & 8.4 B & 9.4 B & 10.2 B \\ \hline Higher than expected & 27 & 8.0 B & 7.8 B & 8.2 B & 8.8 B & 7.6 C \\ \hline \hline \end{tabular} Difference in capital letters indicate significant difference (\(p\) \(<\) 0.05) in group means.  
\end{table}
Table 3: NASA TLX subscales by perceived trust levels.  
The evaluation of the subscales showed patterns different from the overall workload (see Figure 5). The effects of automation statuses were notable, while the effects of automation levels were mixed. Only perceived performance (\(F\) (3, 187) = 5.26, \(p\) \(<\) 0.001) and frustration (\(F\) (3, 187) = 11.879, \(p\) \(<\) 0.001) demonstrated significant effects of automation levels, while for the mental demand (\(F\) (3, 189) = 2.88, \(p\) = 0.698) and time demand (\(F\) (3, 189) = 2.78, \(p\) = 0.708) subscales, the automation level did not impact the measurements in Failed automation. Similarly, for the effort subscale, the participants did not recognize the effect of automation level in both Routine and Failed automations (\(F\) (3, 189) = 2.82, \(p\) = 0.703; \(F\) (3, 189) = 2.88, \(p\) = 0.698).  
The correlation between subscales and overall scores shows that the subscales are highly interrelated, while perceived performance is inversely correlated to other subscales and overall workload. The overall workload was highly correlated with mental demand and temporal demand (r = 0.78 and r = 0.63, respectively). In addition, temporal demand and mental demand were highly correlated (r = 0.65).  
Participants reported a perceived trust in automation level as expected or higher than expected for 84% with the remaining 16% reported as lower than expected (see Table 3). Perceived trust is well correlated with participants' subjective workload scales (r = 0.79), and they also reported poorer perceived performance with unexpectedly medium (as expected) perceived trust on automation (\(p\) \(<\) 0.01). Frustration differed statistically across the three difficulty expectation levels, with frustration lowest when the perceived trust level was higher than expected (\(p\) \(<\) 0.001).