## 2 Materials and Methods
### Assessment of Impact of the Usability Experience  
The collected data about usability have a positive (for example, comfort perception) or negative (for example, discomfort perception) valence. In both cases, it is necessary to evaluate their impact on the system and their relationship with the user, specifically with respect to usability. Impact can be also interpreted as the influence of the system on personal behavior, social activity, or economic value. In a positive experience, we can define \"impact\" as an assessment of the level of satisfaction in the utilization and experience of a system by the user. In the case of a positive experience, the participant experiences no or minimal problems for the successful completion of the task. Typical examples of these impacts could be the participant's willingness to buy the system, their intention to increase their use of the system, the possible adoption of new behaviors in daily life to accommodate the system, or the eventual promotion of the system in a group of friends/relatives. These factors could be rated over a five-point Likert scale, indicating the different levels of impact: high, moderate, low, or null when there is a neutral evaluation or no influence on user experience. In a negative experience, we can define \"impact\" as the assessment of the severity and frequency of the problems encountered during the use and experience of a system by the user and the level of influence of this problem on successful task completion. To identify and prioritize the eventual recommendations for the improvement of a system, it is important to adopt a method for the classification of the impact of a problem emerging from the analysis of the data collected during the evaluation activities. This method must consider the combination of two main factors: the severity (\(S\)) of the problem and the frequency (\(F\)) of occurrence or the number of users experiencing critical or non-critical problems during the evaluation (\(N_{c}+N_{nc}\)). In the first approach, the frequency of occurrence of problems or critical errors or non-critical errors during the execution of a given task is simply computed as the number of these errors (\(N_{c}\) and \(N_{nc}\)) or the ratio between the number of errors and the time to task completion (\(ttc\)). In this case, the impact I is computed as:  
\[I=S\times F=\frac{S_{c}\times N_{c}+S_{nc}\times N_{nc}}{ttc} \tag{1}\]  
where \(S\) is the severity, \(F\) is the frequency, \(S_{c}\) is the severity of critical errors, \(N_{c}\) is the number of critical errors, \(S_{nc}\) is the severity of non-critical errors, \(N_{nc}\) is the number of non-critical errors, and ttc is the time to task completion. Instead, considering users experiencing errors, the frequency of users is the ratio (or percentage) between the number of subjects experiencing critical (\(n_{c}\)) or non-critical errors (\(n_{nc}\)) and the total number of participants (\(n_{tot}\)). So, following this approach, we can define the impact \(I\) as:  
\[I=\frac{S_{c}\times n_{c}+S_{nc}\times n_{nc}}{n_{tot}} \tag{2}\]  
where \(S_{c}\) is the severity of critical errors, \(n_{c}\) is the number of participants experiencing critical errors, \(S_{nc}\) is the severity of non-critical errors, \(n_{nc}\) is the number of participants experiencing non-critical errors, and \(n_{tot}\) is the total number of participants. According to these two factors (severity and frequency), we can distinguish four levels of negative impact:  
* High, i.e., the presence of problems that prevent the user from completing the task (critical error) and their number and/or frequency;
* Moderate, i.e., the presence and high frequency of situations that cause difficulty to the user, but she/he succeeds in completing the task (non-critical error with medium impact);
* Low, i.e., the occurrence and low frequency of repetition of minor problems that do not significantly affect the completion of the task (non-critical error with low impact);
* Null, i.e., the absence of any negative conditions.