Human factors practitioners can come from a variety of backgrounds; though predominately they are Psychologists (Cognitive, Perceptual, and Experimental) and Engineers. Designers (Industrial, Interaction, and Graphic), Anthropologists, and Computer Scientists also contribute. Where as ergonomics tends to focus on the anthropometrics for optimal human-machine interaction, human factors is more focused on the cognitive and perceptual factors. Areas of interest for human factors practioners include the following: workload, fatigue, situational awareness, usability, learnability, attention, vigilance, human performance, control and display design, stress, visualization of data, individual differences, aging, accesibility, shift work, work in extreme environments, and human error. Human Factors is only one element of the broad category of Critical Systems Research, in which human performance (accurate, difficult and error-prone) and machine performance (accurate, difficult and error-prone), are critically studied to reveal patterns of outcome that can be labelled as 'successful,' 'somewhat reliable,' 'error-prone' or 'failing completely.' When systems, both biological and mechanical/artificial, reach system critical conditions they fail to operate as intended by humans. This moment of criticality defines the upper limit of the system's functional utility. Study of critical systems helps humans find ways of improving their desired function and outcomes. Both human factors and ergonomics fall into subset studies within the study of critical systems. Critical systems also include fields of study that are not necessarily man-machine interaction specific (such as linguistics, medicine, the law, business management and all applied fields of study). Failures in these areas often prejudices direct man-machine interaction.