## 5 Results and Discussion
### Cognitive Assessment  
As detailed in Section 3, the cognitive workload assessment combines the operators' perceived evaluation with the biometric monitoring. This allows a more reliable assessment to be obtained, based on both subjective and objective measures, in order to avoid partial and untrustworthy considerations. For this reason, the methodology weighs the collected physiological parameters with the results of the self-assessment questionnaires, for each phase. The first analysis focused on the results of the subjective evaluations, through NAS and NASA-TLX, to understand the perceived workload related to the different phases of the trials independently by the variations in their physiological parameters during the execution of the tasks. The NAS scale has been answered before the execution of the tasks (NAS0) to record the basal level of perceived stress, immediately after each of the three different test phases (NAS1, NAS2, NAS3) to distinguish between eventual different stress levels, and at the end of the test, after a restful period (NAS4) to verify that the perceived stress came back to the basal level. Notably, the first trial phase involves a more stressful execution forced by a time limit, and in the second phase an increased mental effort is compelled by the secondary task (i.e., counting backward), while in the third phase the tasks are executed without additional stressors. In Figure 7 the trend of the perceived stress by NAS, averaged over all the operators, is shown. The perceived stress increases from the beginning of phase 1 to the end of phase 2, and then decreases until the end of phase 3. Even if the protocol supposed that the first phase is the most stressful one in a technical sense, operators on average felt more stressed during the second one. This is probably due to the weak border, from a user perspective, between stress and mental load. Indeed, the need to correctly perform two different operations (instead of one), leads to an increment of the mental demand, as well as of stress and fatigue contemporarily. Consequently, stress perception decreases in phase 3 (NAS3 is lower than NAS1) and drops down under the initial basal level at the end of phase 3 (NAS4 < NAS0 likely due to the fact that performance anxiety is perceived at the beginning of testing). Similarly, the results from NASA-TLX were analyzed as average values on all operators, for each phase and for the six different domains within the NASA-TLX, as shown in Figure 8.  
Figure 7: NAS average results (with SD). The mental demand, as expected, is widely higher in phase 2. Moreover, phase 2 also showed higher perceived effort and frustration with respect to the other phases. Therefore, the cognitive and emotional involvements are higher when a dual task (i.e., executing the task and counting) is asked to be executed. However, the necessity to obtain a good performance is greater in phase 3, probably as the final result of the engine oil filter replacement is fulfilled in this phase. Concerning frustration, this is higher in phase 1 due to the countdown, while the perceived effort is greater in phase 3 probably as a result of an accumulation effect. Similarly, even the physical demand constantly increases from phase 1 to phase 3, due to the passing of time doing similar actions. The highest perception of temporal demand during phase 1 is obviously due to the imposed time limit for task execution. Figure 9 shows the S and MW metrics, computed with the physiological parameters as presented in Section 3. As foreseen by the protocol, a higher stress level should be measured in phase 1, and a greater mental workload in phase 2. Instead, the algorithm reveals that both stress and  
Figure 8: NASA-TLX average results. Figure 9: Mental workload and Stress average scores on test phases. mental workload are comparable in phases 1 and 3, while they are both higher in phase 2, during the execution of the dual task. Indeed, the physiological response to a stimulus does not have a univocal meaning.