The Human Component
THE HUMAN COMPONENT 2-03
TABLE 1. FUNCTIONAL ADVANTAGES AND DISADVANTAGES OF MEN AND MACHINES
Data Sensing Man
Can monitor low-probability events for which, because of the number pos-sible, automatic systems would not be feasible.
Under favorable conditions absolute thresholds of sensitivity in various modes are very low.
Can detect masked signals effectively in an overlapping noise spectrum on displays such as radar and sonar.
Able to acquire and report information incidental to primary activity.
Not subject to jamming by ordinary methods.
Machines
Program complexity and alternatives limited so that unexpected events
Discovery and selection of incidental intelligence not feasible in present de-signs.
Generally subject to disruption by various interference and noise sources.
Data Processing
Reasonable reliability in which the same purpose can be accomplished by different approach (corollary of re-programming ability).
Can make inductive decisions in situa-tions not previously encountered; can generalize from few data.
Computation is weak and relatively in-accurate; optimal theory of games strategy cannot be routinely expected.
Channel capacity limited to relatively small information throughput rates.
Can handle variety of transient over-loads and some permanent overover-loads without disruption.
Short-term memory relatively poor.
May have high reliability at increased cost and complexity. Particularly re-liable for routine repetitive func-tioning.
Transient and permanent overloads may lead to disruption of system.
Short-term memory and access times excellent.
( continued)
2-04 SYSTEMS ENGINEERING
TABLE 1. FUNCTIONAL ADVANTAGES AND DISADVANTAGES OF MEN AND MACHINES-( Continued)
Data Transmitting Man
Can tolerate only relatively low im-posed forces and generate relatively low forces for short time periods.
Generally not good at tracking though may be satisfactory where situation re-quires frequent reprogramming; can change to meet situation. Is best at position tracking with changes under 3 radians per second.
Performance may deteriorate with time; usually recovers with rest.
Relatively high response latency.
Machines
Can withstand very large forces and generate them for prolonged periods.
Good tracking characteristics may be obtained over limited set of require-ments.
Behavior decrement relatively small with time; wear maintenance and product quality control necessary.
Arbitrarily low response latencies possible.
Economic Properties
Relatively inexpensive for available Complexity and supply limited by cost complexity and in good supply; must and time; performance built in.
be trained.
Light in weight and small in size for function achieved; low power require-ment, less than 100 watts.
Easy to maintain with minimum of "in task" extras.
N onexpendable and interested in per-sonal survival; emotional.
Equivalent complexity and function would require radically heavier compo-nents and enormous power and cooling resources.
Maintenance problem becomes dis-proportionately serious as complexity increases.
Expendable and unconscious of per-sonal existence; will perform without distraction from problems arising out-side of task.
2. DESIGN PROBLEMS SPECIFIC TO HUMAN COMPONENTS
Once the task has been defined, it is necessary to consider the environ-ment in detail to insure that the assignenviron-ment can and will be fulfilled. The important physical aspects are shown in Fig. 2, but the environment has not been completely specified without th~ information .input and output coupling to and from the man (Ref. 20). The task must be analyzed into decisions, and assurance provided to the operator that the required information can and will be received as it is needed. He must also have
THE HUMAN COMPONENT 2-05 Environment
Thermal
~
AtmosphericIf?
Sensory display
Man Machine
Motor output
2Jfl 4> ,
Visible Ionizing Mechanical
radiation radiation
FIG. 2. Environmental and machine links to the human.
efficient means for coupling his control actions to the system. Specific hardware design problems requiring data on humans fall into the following ca tegories :
1. Problems of the general working environment such as optimum heat-mg, lighting, and ventilation in relation to human physiological param-eters.
2. Problems of size, shape, and arrangement in which human character-istics limit the physical form of controls and spaces.
3. Problems of information input in "which human encoding properties limit channel capacity in terms of speed and sensitivity of response and the nature of the stimulating energy which can be applied to the senses.
4. Problems of motor output (including voice) in which human neuro-muscular characteristics limit the static and dynamic properties of con-trol movements and the useful power that is available.
In this presentation attention will be directed primarily to categories 3 and 4.
3. INFORMATION JNPUTS TO THE HUMAN COMPONENT
Displays and instruments are devices which enable adequate control by virtue of the information they transmit. The operator is a channel for processing and transmitting this information from the display to the con-trols. The controls are monitored by means of an information channel from the operator to the machine. The channel capacity of the human
2-06 SYSTEMS ENGINEERING
and its optimum utilization through proper encoding of the information to be transmitted by the human to the controls is an orienting view-point that aids in making appropriate design decisions.
Vision
Perceptual Capacity. Vision is the most important and widely utilized input channel. In considering the visual acuity and physical character-istics of the human eye, it has been estimated that about 4.3 million bits per second can be received by the retina (Ref. 16). However, this is not perception, the usable acceptance of data. Empirical studies have re-vealed that the maximum amount of information which can be accepted by a human observer when he attempts to locate a point on a straight line (e.g., an indicator scale) is about 3 bits (Ref. 9). As the number of coordinates is increased, there is an increase in the message carrying ca-pacity of the stimulus, but this does not appear to be linear. Thus two coordinates of a dot on a plane transmit about 4.4 bits and the eight coordinates of one-to-four dots on a plane about 7.8 bits (Ref. 17). The importance of the type of encoding is further suggested by the fact that apparently maximum perceptual capacity is approached during the read-ing of printed English text. This figure is about 50 bits per second (Ref.
3) .
Other Limitations. The time operating characteristics of the eye im-pose certain communication channel limitations. Dark and light adapta-tion (Ref. 13) of the light-sensitive material of the retina must be con-sidered in any evaluation of the communication constraints caused by the physical environment. Furthermore, the stimulus brightness is perceived in relation to its previous intensity level. This function is approximated logarithmically over the usual range of values (Refs. 23, 28). The eye scans the visual field in a series of irregular discrete steps. This saccadic motion limits the fixation time to about 90% of the observation time in-terval (Ref. 3). This motion is not random and, although little is known about the causal relations, it appears quite efficient as a preliminary pat-tern discrimination filter (Ref. 19).
Filter Action. Perceived data are recognized as part of a time-series and experiments have shown (Ref. 10) that the human operator appar-ently cannot perceive data without some reference to previous and pos-sible future data. The observer appears to postulate a structure on the incoming message and proceeds to verify or deny the hypothesized pat-tern. The hypothesized pattern introduces a certain amount of redun-dancy and thus can only diminish the channel capacity. His perceptual operation thus becomes a filter operation which examines the redundancy of the received data in comparison to that of the hypothesis. For
ex-THE HUMAN COMPONENT 2-07