## 3 Approach
### Human Skills Modeling Engine (Data-Driven)
#### 3.2.1 Multi-Attribute Skill Matrix  
In manufacturing systems, skill matrices are common to plan and manage skills for jobs, teams, projects, or departments. It is a tool used to track and rank employees' skills. This framework helps to select the right people for a job and to identify missing competencies or gaps between the staff. This procedure allows the employer to keep track of their employee's development. The conventional skill matrix of operators in production systems is generally made in a single table with the workstations on one side as rows and the workers in the columns. These stations have some operations that describe the process, and the skill level of the worker represents the ability that this person must have to carry out the work in the workstation [51]. The skill matrix proposed in this paper implies adding one more dimension to the conventional skill matrix. The skill levels do not refer to the workstations per worker,  
Figure 4: Human skills modeling engine. but rather to the skills. This matrix is composed of different sheets regarding the skills to evaluate (e.g., grasping, assembling, visual inspection, etc.). Thus, the worker will have a skill level for each skill per workstation. This way, the performance is evaluated in more detail. This matrix definition is explained in Figure 5.  
To develop every sheet of the skill matrix, the first step is to identify the skills or attributes that define the production process. Then, create specific training content (material, type, method, and duration) for each skill to be able to train the new workers or improve the skill levels of the current workforce. As previously mentioned, the skill matrix workstations are related to the skills by a process description (list of tasks). Figure 6 shows how to establish this relationship and the skill matrix flow. In this way, each workstation has a process description made up of simple and individual operations, which will be included in one of the skills defined in the skill matrix definition. For example, there is a workstation A in the assembly line of the main part called C. The skills defined in the skill matrix definition are grasping, gluing, screwing, assembling, welding, and visual inspection. Workstation A is composed of the following tasks: grasping object B, assembling B into C, and screwing top left object B. These individual operations are included within the skills: grasping, assembling and screwing. Figure 5: Skill matrix definition. [Source: inspired by the chapter “Skill management” (pages 113–119) in reference [12]]. Figure 6: Skill matrix flow. [Source: inspired by the chapter “Skill management” (pages 113–119) in reference [12]]. Following the flow presented in Figure 6, the next step is to elaborate or review the skill matrix by identifying the skill level of the worker for each skill, and the training plan (monthly, annual, etc.). After that, the operator should perform the training and the supervisor should review the multi-attribute skill matrix every six/twelve months to keep it up to date. Figure 7 presents the method to identify the current skill level of the worker. Workers should be audited following a checklist created per skill and the levels are ranked:  
* Skill level 0. The worker is not required to learn this skill. * Skill level 1. The worker has gone through the training and knows the basic principles of the skill. * Skill level 2. The worker has gone through the training and has demonstrated the basic principles learned but needs occasional support. * Skill level 3. The worker has gone through the training and has demonstrated the skill of working alone without supervision. * Skill level 4. The worker has demonstrated the skill daily and is able to train others. After the skill level is defined, it should be reviewed with the supervisor. The multi-attribute skill matrix is stored in the database. Isolated, it does not trigger any change in the human skills modeling engine.