## 3 The Proposed Method for Operators' Workload Assessment  
The proposed assessment method has been designed to support the design of every kind of industrial product, process, or environment: it allows the simultaneous analysis of both the physical and cognitive workload of operators while performing habitual tasks during the work shift. The method has been defined to be cause the minimum amount of intrusion as possible for the operator: this involves a minimum set of non-invasive wearable devices for the monitoring of motions and physiological parameters, and only two questionnaires for the subjective self-assessment of cognitive conditions. In fact, if the set-up is not properly defined, this may influence the physical and cognitive perception, altering the overall user experience of the operator. The user is asked to wear a set of trackers to enable motion capture for postural and activity analysis, together with an eye-tracker device for eye activity monitoring, and a bracelet for biometric monitoring. Motion capture allows a physical workload analysis, while the two latter devices (i.e., eye tracking and bracelet) are able to collect different kinds of physiological parameters, such as heart rate (HR), inter-beat interval (IBI, RR), electrodermal activity (_EDA_), pupil diameter (_PD_), that can be properly combined to monitor the cognitive workload, and to detect eventual mental overload and stressful conditions. To strengthen the reliability of the cognitive assessment, the method also includes the administration of questionnaires (NAS and NASA-TLX) for the self-assessment of the perceived cognitive and emotional conditions, before and after the execution of the tasks. NAS is a unidimensional scale that consists of a line divided into 10 intervals, numbered from 0 to 10, where 0 indicates the absence of perceived stress, and 10 the presence of a very strong stress. The worker is asked to select the whole number (0-10 integers) that best reflects the intensity of the perceived stress immediately after the tasks' execution. Also, the scale must be answered before starting the tasks to record the basal level of perceived stress, and at the end of the procedure, after a brief period of rest to verify that the stress perception came back to the basal level. The NAS questionnaire has been selected among other questionnaires as studies suggest it as a valid, effective, and easy-to-implement tool for the rapid assessment of perceived stress in the industrial environment. On the other side, the multidimensional NASA-TLX has been chosen to discriminate between different cognitive and emotional states. Indeed, through the six items, it is possible to evaluate not only the perceived mental demand, but also the feelings of frustration, effort, and the performance. The worker is asked to fulfil the questionnaire at the end of the task execution. Moreover, the assessment method includes the analysis of the performances to be correlated with the cognitive conditions. By video recording and reporting the actions of the operators, it is possible to analyze the execution times and the committed errors, which may derive from unbalanced mental workload. The proposed assessment method is based on the combined analysis referring to the postural risk, the mental workload, and the stress. Different classes of parameters are considered:  
1. Human body segment position and motions into the three-dimensional space, collected by the motion capture systems;
2. Pupil diameter collected by the eye tracking system;
3. Physiological parameters referring to the user's cardiovascular activity (i.e., HR and RR) and skin conductance (i.e., _EDA_), collected by the biometric wearable device;
4. Subjective assessment based on NAS and NASA-TLX questionnaires;
5. Performance data like execution time, collected by video analysis. A proper integration and processing of such parameters can provide an early assessment of the physical effort, the mental workload and the perceived stress.