AS IEC 62508:2011 pdf – Guidance on human aspects of dependability.
4.3 Human characteristics
4.3.1 Introductory remark
Human beings have a set of physical, cognitive and psychological characteristics that vary from person to person (4.5.2). These characteristics provide fundamental limitations to the human capabilities that need to be taken into account in systems design. Appropriate training and experience will enable people to work more effectively, but only within their limitations.
Human reliability and performance will be Influenced by the design of the machine and by the physical and social environment (4.5.1). To ensure a working situation with high dependability, the system should be designed so that the stress on the human being due to the work task, work environment and technical design remains within acceptable limits.
4.3.2 Human limitations
The design should take account of human limitations.
a) Physical limitations
• Anthropometric and biomechanecal constraints.
• Sensory constraints (e.g. the range of signals that can be perceived and
b) Cognitive limitations
• The time needed between perception of a signal and an action in response. This can range from a few hundred milliseconds for skillbased actions where response is quasi automatic (and is not reasoned), to several seconds or minutes where reasoning and analysis is necessary.
• Limitations of short-term memory. Only 5 to 7 items of information can be held in short-term memory. For larger amounts of information, mental models or patterns are constructed.
• Limitabons on the amount of information that can be processed at one time (working memory).
• The inability to focus effectively on more than one task at a time or process information in parallel.
• Potential for loss of situational awareness resulting in actions based on incorrect perception of reality.
C) Psychological limitations
• Performance degradation due to physical and mental fatigue or boredom.
• Tendency for decisions and actions to be based on emotional rather than reasoned responses particularly under situations of stress.
Since these characteristics of humans cannot be designed out of the system, the division of tasks between people and the rest of a system and the design of technical systems and interfaces have to be taken into account, The relative strengths of humans and machines should be considered (4.4.3).
4.3.3 Comparison of humans and machines
The allocation of activities and operational steps between human beings and machines should take into account the relative sttengths of humans and machines.
a) Human strengths
• Ablhty to perceive patterns of light or Sound.
• Ability to improvise and use flexible procedures.
• Ability to store very large amounts of information for long periods and to recall relevant
facts at the appropriate time.
• Ability to reason inductively.
• Ability to exercise judgement,
b) Machine stfengths
• Ability to detect small amounts and a wider range of visual and acoustic signals.
• Ability to respond quickly to control signals, and to apply great force smoothly and precisely.
• Ability to perform repetitive and routine tasks consistently and accurately.
• Ability to store information briefly and then to erase it completely.
• Ability to reason deductively, including computational ability.
• Ability to handle highly complex operations and to do many different things at once.
There are major differences between humans and machines
• Machines can be modified, redesigned, and retrofitted whereas humans cannot. Humans are born with innate, genetically determined differences that are shaped by the environment. Innate aptitudes or abilities are developed through education and training.
• Machines can be manufactured to provide exact output and duplicate precise operation. Humans are not identical and vary across all sensory. cognitive, physical and performance characteristics. Specific aspects of human performance can be made more equal through selection and training.
4.4 Human performance shaping factors
4.4.1 General
The performance and reliability of people within a system will vary depending on a range of internal and external conditions that differ from person to person and from one instant to another. The factors that influence the capability of human beings to reliably accomplish a task are called performance shaping factors (also known as the context of use).
Figure 1 indicates the types of performance shaping factors with grey arrows.
Figure 2 provides examples distinguishing between external and internal performance shaping factors.