## 6 Usability Tests  
Usability tests were conducted at a quiet and controlled location within the Department of Informatics, Systems and Communication of the University of Milano-Bicocca. We involved 10 users who tested both the acquisition and the labeling applications by performing and labeling activities for 20 min. The users' ages were around 20, 40, 50, and 60 and were equally distributed, and for their genders, half were female and half were male. The users were trained in the use of both the applications by a moderator who was an expert user of the application. The usability test included four tasks that were conducted individually, that is, one user at a time. After the test, to obtain the users' impressions about the usability of the applications, the users completed a questionnaire. The questionnaire included questions related to the functionalities of the two applications, and the questions were inspired by the System Usability Scale (SUS) questionnaire [50].  
The tasks involved in the tests are described in the following.  
1. Use the acquisition application to acquire the data from the sensors of the smartphone only, and use of the labeling application to label the activities performed following a predefined list of activities. For this task, the moderator instructed the users to follow the predefined list of activities denoted as \"Relax\", which included walking for 300 s, sitting for 60 s, and standing for 120 s.
2. Use of the acquisition application to acquire the data from the sensors of both the smartphone and the smartwatch, and use of the labeling application to label the activities performed following a predefined list of activities. For this task, the moderator instructed the users to follow the predefined list of activities denoted as \"Soft sport\", which included running for 180 s and jumping for 120 s.
3. Use of the acquisition application to acquire the data from the sensors of the smartphone, and use of the labeling application to label the activities performed that were freely chosen from the free choice list. For this task, the moderator instructed the users to choose at most three activities from the free choice list and to perform them for at most 15 s. The free choice list included running, walking, sitting, and going downstairs.  
Figure 8: The information about the sensors used for recording in the smartphone (**a**) and in the smartwatch (**b**).  
Use of the acquisition application to acquire the data from the sensors of both the smartphone and the smartwatch, and use of the labeling application to label the activities performed that were freely chosen from the free choice list. For this task, the moderator gave the same instructions as in the third task above.  
The users were also instructed to move between tasks and to do whatever they would naturally do. The moderator observed all the test sessions and annotated the errors and the problems that occurred and the time taken by each user to complete a task.  
Table 2 reports the results of the questionnaire obtained by taking the median of all the users' votes. The first 10 statements refer to both the applications, while the 13 th and 14 th statements refer to the acquisition and labeling applications, respectively. Generally, the applications evaluated the applications as easy to use. The users employed a very short time to start using the applications correctly, and they declared the use of the applications' interface to be very pleasant. Some users found the action of selecting the sensors to be recorded somewhat difficult when both the smartphone and the smartwatch were selected. For this, additional instructions by the moderator were needed.