## 2 Background
### Cognitive Ergonomics in HRCS  
The cognitive dimension of ergonomics is dedicated, among other subjects, to the analysis of mental workload and human-machine interaction by assessing the cognitive effectiveness of a given task and its employed tools [78]. As mentioned by [79], the cognitive workload is the concrete result of humans interplaying with a task and the resulting difference between the number of mental processing resources dedicated to the execution of a task and the human total resource capacity. In other words, cognitive workload represents the cost repaid by human operators in order to accomplish a determined level of performance [80]. Accordingly, issues in cognitive workload emerge when the mental demand imposed by a task is higher than the total mental capacity of the human operator [78]. Work design strategies to enhance tasks' mental demands and humans' mental capacity can be harnessed likewise (i.e., decreasing the difficulty of a task or training human operators to program a collaborative robot). With the objective to reduce or eliminate cognitive workload issues, numerous techniques for its assessment have been proposed. The following measurement techniques analyze, from different perspectives, the effects that a task might have on a human's cognitive workload:  
* Performance measures: Based on difficulty variations and task-required outcomes. * Objective measures: Based on physiological responses linked to the nervous system. * Subjective measures: Based on direct opinions from the human operator experience. Changes in heart rate, pupil diameter, flickering rate, visual fixation time, brain activity, body temperature, respiratory frequency, and hormonal levels are some physiological responses associated with cognitive workload variation. Nevertheless, in the context of HRCS, sensors that measure these responses should not be invasive for human operators as their usage might be impractical or non-ergonomic within industrial settings, as described in ISO 26800:2011 [81]. Conversely, subjective measures can be divided into unidimensional and multidimensional, depending on the scope of the analysis. A general unique measurement of workload can be rapidly attained from unidimensional techniques, whereas diagnostic information about the system and the specific sources of workload are the resultants of multidimensional methods [79]. Subjective measurements offer the flexibility of collecting data while human operators are performing a task or after it has been completed, so their experience is still vivid [82]. Moreover, tests can be applied in manual, digital, or vocal channels, making them not invasive for both laboratory and industrial environments. Relevant subjective measurements found in the cognitive ergonomics literature are listed below:  
* Bedford Workload Scale (BWS): Unidimensional rating technique that allows us to deliberate if more activities can be added to a task or if their number should be reduced as human operators only have a limited amount of cognitive resources available [83]. The Bedford Workload Scale uses a hierarchical decision tree to globally rate workload and it is normally applied within operational validating phases of the engineering life cycle. Similar unidimensional scales are the Cooper-Harper Scale (MCH) and the Subjective Workload Dominance Technique (SWORD). * NASA-Task Load Index (TLX): Multi-dimensional rating technique that possesses solid diagnostic properties within design testing phases of the engineering life cycle. The overall workload is estimated from the weighted average of ratings that participants give to the following six factors: mental demand, physical demand, and temporal demand (task-related factors); effort and performance (behavior-related factors); and frustration (subject-related factors). The NASA-TLX individual factors were established, henceforth, as relevant independent contributors of workload nature and its associated sources [84].