Karl H. E. Kroemer

In what follows, three of the most important concerns of ergonomic design will be examined: first, that of controls, devices to transfer energy or signals from the operator to a piece of machinery; second, indicators or displays, which provide visual information to the operator about the status of the machinery; and third, the combination of controls and displays in a panel or console.

Designing for the Sitting Operator

Sitting is a more stable and less energy-consuming posture than standing, but it restricts the working space, particularly of the feet, more than standing. However, it is much easier to operate foot controls when sitting, as compared to standing, because little body weight must be transferred by the feet to the ground. Furthermore, if the direction of the force exerted by the foot is partly or largely forward, provision of a seat with a backrest allows the exertion of rather large forces. (A typical example of this arrangement is the location of pedals in an automobile, which are located in front of the driver, more or less below seat height.) Figure 1 shows schematically the locations in which pedals may be located for a seated operator. Note that the specific dimensions of that space depend on the anthropometry of the actual operators.

Figure 1. Preferred and regular workspace for feet (in centimetres)

The space for the positioning of hand-operated controls is primarily located in front of the body, within a roughly spherical contour that is centred at either the elbow, at the shoulder, or somewhere between those two body joints. Figure 2 shows schematically that space for the location of controls. Of course, the specific dimensions depend on the anthropometry of the operators.

Figure 2. Preferred and regular workspace for hands (in centimetres)

The space for displays and for controls that must be looked at is bounded by the periphery of a partial sphere in front of the eyes and centred at the eyes. Thus, the reference height for such displays and controls depends on the eye height of the seated operator and on his or her trunk and neck postures. The preferred location for visual targets closer than about one metre is distinctly below the height of the eye, and depends on the closeness of the target and on the posture of the head. The closer the target, the lower it should be located, and it should be in or near the medial (mid-sagittal) plane of the operator.

It is convenient to describe the posture of the head by using the “ear-eye line” (Kroemer 1994a) which, in the side view, runs through the right ear hole and the juncture of the lids of the right eye, while the head is not tilted to either side (the pupils are at the same horizontal level in the frontal view). One usually calls the head position “erect” or “upright” when the pitch angle P (see figure 3) between the ear-eye line and the horizon is about 15°, with the eyes above the height of the ear. The preferred location for visual targets is 25°–65° below the ear-eye line (LOSEE in figure 3), with the lower values preferred by most people for close targets that must be kept in focus. Even though there are large variations in the preferred angles of the line of sight, most subjects, particularly as they become older, prefer to focus on close targets with large LOSEE angles.

Figure 3. Ear-eye line

Designing for the Standing Operator

Pedal operation by a standing operator should be seldom required, because otherwise the person must spend too much time standing on one foot while the other foot operates the control. Obviously, simultaneous operation of two pedals by a standing operator is practically impossible. While the operator is standing still, the room for the location of foot controls is limited to a small area below the trunk and slightly in front of it. Walking about would provide more room to place pedals, but that is highly impractical in most cases because of the walking distances involved.

The location for hand-operated controls of a standing operator includes about the same area as for a seated operator, roughly a half sphere in front of the body, with its centre near the shoulders of the operator. For repeated control operations, the preferred part of that half sphere would be its lower section. The area for the location of displays is also similar to the one suited to a seated operator, again roughly a half sphere centred near the operator’s eyes, with the preferred locations in the lower section of that half sphere. The exact locations for displays, and also for controls that must be seen, depends on the posture of the head, as discussed above.

The height of controls is appropriately referenced to the height of the elbow of the operator while the upper arm is hanging from the shoulder. The height of displays and controls that must be looked at is referred to the eye height of the operator. Both depend on the operator’s anthropometry, which may be rather different for short and tall persons, for men and women, and for people of different ethnic origins.

Foot-operated Controls

Two kinds of controls should be distinguished: one is used to transfer large energy or forces to a piece of machinery. Examples of this are the pedals on a bicycle or the brake pedal in a heavier vehicle that does not have a power-assist feature. A foot-operated control, such as an on-off switch, in which a control signal is conveyed to the machinery, usually requires only a small quantity of force or energy. While it is convenient to consider these two extremes of pedals, there are various intermediate forms, and it is the task of the designer to determine which of the following design recommendations apply best among them.

As mentioned above, repeated or continual pedal operation should be required only from a seated operator. For controls meant to transmit large energies and forces, the following rules apply:

• Locate pedals underneath the body, slightly in front, so that they can be operated with the leg in a comfortable position. The total horizontal displacement of a reciprocating pedal should normally not exceed about 0.15 m. For rotating pedals, the radius should also be about 0.15 m. The linear displacement of a switch-type pedal may be minimal and should not exceed about 0.15 m.

• Pedals should be so designed that the direction of travel and the foot force are approximately in the line extending from the hip through the ankle joint of the operator.

• Pedals that are operated by flexion and extension of the foot in the ankle joint should be so arranged that in the normal position the angle between the lower leg and the foot is approximately 90°; during operation, that angle may be increased to about 120°.

• Foot-operated controls that simply provide signals to the machinery should normally have two discrete positions, such as ON or OFF. Note, however, that tactile distinction between the two positions may be difficult with the foot.

Selection of Controls

Selection among different sorts of controls must be made according to the following needs or conditions:

• Operation by hand or foot

• Amounts of energies and forces transmitted

• Applying “continuous” inputs, such as steering an automobile

• Performing “discrete actions,” for example, (a) activating or shutting down equipment, (b) selecting one of several distinct adjustments, such as switching from one TV or radio channel to another, or (c) carrying out data entry, as with a keyboard.

The functional usefulness of controls also determines selection procedures. The main criteria are as follows:

• The control type shall be compatible with stereotypical or common expectations (for instance, using a push-button or toggle switch to turn on an electric light, not a rotary knob).

• Size and motion characteristics of the control shall be compatible with stereotypical experience and past practice (for instance, providing a large steering wheel for the two-handed operation of an automobile, not a lever).

• The direction of operation of a control shall be compatible with stereotypical or common expectations (for instance, an ON control is pushed or pulled, not turned to the left).

• Hand operation is used for controls that require small force and fine adjustment, while foot operation is suitable for gross adjustments and large forces (however, consider the common use of pedals, particularly accelerator pedals, in automobiles, which does not comply with this principle).

• The control shall be “safe” in that it cannot be operated inadvertently nor in ways that are excessive or inconsistent with its intended purpose.

Table 1. Control movements and expected effects

Blank: Not applicable; + Most preferred; – less preferred. ¹ With trigger-type control. ² With push-pull switch. ³ Up in the United States, down in Europe.

Source: Modified from Kroemer 1995.

Table 1 and table 2 help in the selection of proper controls. However, note that there are few “natural” rules for selection and design of controls. Most current recommendations are purely empirical and apply to existing devices and Western stereotypes.

Table 2. Control-effect relations of common hand controls

Blank: Not applicable; +: Most preferred; –: Less preferred; = Least preferred. ¹ Estimated (no experiments known).

Source: Modified from Kroemer 1995.

Figure 4 presents examples of “detent” controls, characterized by discrete detents or stops in which the control comes to rest. It also depicts typical “continuous” controls where the control operation may take place anywhere within the adjustment range, without the need to be set in any given position.

Figure 4. Some examples of "detent" and "continuous" controls

The sizing of controls is largely a matter of past experiences with various control types, often guided by the desire to minimize the needed space in a control panel, and either to allow simultaneous operations of adjacent controls or to avoid inadvertent concurrent activation. Furthermore, the choice of design characteristics will be influenced by such considerations as whether the controls are to be located outdoors or in sheltered environments, in stationary equipment or moving vehicles, or may involve the use of bare hands or of gloves and mittens. For these conditions, consult readings at the end of the chapter.

Several operational rules govern the arrangement and grouping of controls. These are listed in table 3. For more details, check the references listed at the end of this section and Kroemer, Kroemer and Kroemer-Elbert (1994).

Table 3. Rules for arrangement of controls

Source: Modified from Kroemer, Kroemer and Kroemer-Elbert 1994.
Reproduced by permission of Prentice-Hall. All rights reserved.

Preventing Accidental Operation

The following are the most important means to guard against inadvertent activation of controls, some of which may be combined:

• Locate and orient the control so that the operator is unlikely to strike it or move it accidentally in the normal sequence of control operations.

• Recess, shield or surround the control by physical barriers.

• Cover the control or guard it by providing a pin, a lock or other means that must be removed or broken before the control can be operated.

• Provide extra resistance (by viscous or coulomb friction, by spring-loading or by inertia) so that an unusual effort is required for actuation.

• Provide a “delaying” means so that the control must pass through a critical position with an unusual movement (such as in the gear shift mechanism of an automobile).

• Provide interlocking between controls so that prior operation of a related control is required before the critical control can be activated.

Note that these designs usually slow the operation of controls, which may be detrimental in case of an emergency.

Data Entry Devices

Nearly all controls can be used to enter data on a computer or other data storage device. However, we are most used to the practice of using a keyboard with push-buttons. On the original typewriter keyboard, which has become the standard even for computer keyboards, the keys were arranged in a basically alphabetic sequence, which has been modified for various, often obscure, reasons. In some cases, letters which frequently follow each other in common text were spaced apart so that the original mechanical type bars might not entangle if struck in rapid sequence. “Columns” of keys run in roughly straight lines, as do the “rows” of keys. However, the fingertips are not aligned in such manners, and do not move in this way when digits of the hand are flexed or extended, or moved sideways.

Many attempts have been made over the last hundred years to improve keying performance by changing the keyboard layout. These include relocating keys within the standard layout, or changing the keyboard layout altogether. The keyboard has been divided into separate sections, and sets of keys (such as numerical pads) have been added. Arrangements of adjacent keys may be changed by altering spacing, offset from each other or from reference lines. The keyboard may be divided into sections for the left and right hand, and those sections may be laterally tilted and sloped and slanted.

The dynamics of the operation of push-button keys are important for the user, but are difficult to measure in operation. Thus, the force-displacement characteristics of keys are commonly described for static testing, which is not indicative of actual operation. By current practise, keys on computer keyboards have fairly little displacement (about 2 mm) and display a “snap-back” resistance, that is, a decrease in operation force at the point when actuation of the key has been achieved. Instead of separate single keys, some keyboards consist of a membrane with switches underneath which, when pressed in the correct location, generate the desired input with little or no displacement felt. The major advantage of the membrane is that dust or fluids cannot penetrate it; however, many users dislike it.

There are alternatives to the “one key-one character” principle; instead, one can generate inputs by various combinatory means. One is “chording”, meaning that two or more controls are operated simultaneously to generate one character. This poses demands on the memory capabilities of the operator, but requires the use of only very few keys. Other developments utilize controls other than the binary tapped push button, replacing it by levers, toggles or special sensors (such as an instrumented glove) which respond to movements of the digits of the hand.

By tradition, typing and computer entry have been made by mechanical interaction between the operator’s fingers and such devices as keyboard, mouse, track ball or light pen. Yet there are many other means to generate inputs. Voice recognition appears one promising technique, but other methods can be employed. They might utilize, for example, pointing, gestures, facial expressions, body movements, looking (directing one’s gaze), movements of the tongue, breathing or sign language to transmit information and to generate inputs to a computer. Technical development in this area is very much in flux, and as the many nontraditional input devices used for computer games indicate, acceptance of devices other than the traditional binary tap-down keyboard is entirely feasible within the near future. Discussions of current keyboard devices have been provided, for example, by Kroemer (1994b) and McIntosh (1994).

Displays

Displays provide information about the status of equipment. Displays may apply to the operator’s visual sense (lights, scales, counters, cathode-ray tubes, flat panel electronics, etc.), to the auditory sense (bells, horns, recorded voice messages, electronically generated sounds, etc.) or to the sense of touch (shaped controls, Braille, etc.). Labels, written instructions, warnings or symbols (“icons”) may be considered special kinds of displays.

The four “cardinal rules” for displays are:

1. Display only that information which is essential for adequate job performance.
2. Display information only as accurately as is required for the operator’s decisions and actions.
3. Present information in the most direct, simple, understandable and usable form.
4. Present information in such a way that failure or malfunction of the display itself will be immediately obvious.

The selection of either an auditory or visual display depends on the prevailing conditions and purposes. The objective of the display may be to provide:

• historical information about the past state of the system, such as the course run by a ship

• status information about the current state of the system, such as the text already input into a word processor or the current position of an airplane

• predictive information, such as on the future position of a ship, given certain steering settings

• instructions or commands telling the operator what to do, and possibly how to do it.

A visual display is most appropriate if the environment is noisy, the operator stays in place, the message is long and complex, and especially if it deals with the spatial location of an object. An auditory display is appropriate if the workplace must be kept dark, the operator moves around, and the message is short and simple, requires immediate attention, and deals with events and time.

Visual Displays

There are three basic types of visual displays: (1)The check display indicates whether or not a given condition exists (for example a green light indicates normal function). (2)The qualitative display indicates the status of a changing variable or its approximate value, or its trend of change (for example, a pointer moves within a “normal” range). (3) The quantitative display shows exact information that must be ascertained (for example, to find a location on a map, to read text or to draw on a computer monitor), or it may indicate an exact numerical value that must be read by the operator (for example, a time or a temperature).

Design guidelines for visual displays are:

• Arrange displays so that the operator can locate and identify them easily without unnecessary searching. (This usually means that the displays should be in or near the medial plane of the operator, and below or at eye height.)

• Group displays functionally or sequentially so that the operator can use them easily.

• Make sure that all displays are properly illuminated or illuminant, coded and labelled according to their function.

• Use lights, often coloured, to indicate the status of a system (such as ON or OFF) or to alert the operator that the system, or a subsystem, is inoperative and that special action must be taken. Common meanings of light colours are listed in figure 5. Flashing red indicates an emergency condition that requires immediate action. An emergency signal is most effective when it combines sounds with a flashing red light.

Figure 5. Colour coding of indicator lights

For more complex and detailed information, especially quantitative information, one of four different kinds of displays are traditionally used: (1) a moving pointer (with fixed scale), (2) a moving scale (with fixed pointer), (3) counters or (4) “pictorial” displays, especially computer-generated on a display monitor. Figure 6 lists the major characteristics of these display types.

Figure 6. Characteristics of displays

It is usually preferable to use a moving pointer rather than a moving scale, with the scale either straight (horizontally or vertically arranged), curved or circular. Scales should be simple and uncluttered, with graduation and numbering so designed that correct readings can be taken quickly. Numerals should be located outside the scale markings so that they are not obscured by the pointer. The pointer should end with its tip directly at the marking. The scale should mark divisions only so finely as the operator must read. All major marks should be numbered. Progressions are best marked with intervals of one, five or ten units between major marks. Numbers should increase left to right, bottom to top or clockwise. For details of dimensions of scales refer to standards such as those listed by Cushman and Rosenberg 1991 or Kroemer 1994a.

Starting in the 1980s, mechanical displays with pointers and printed scales were increasingly replaced by “electronic” displays with computer-generated images, or solid-state devices using light-emitting diodes (see Snyder 1985a). The displayed information may be coded by the following means:

• shapes, such as straight or circular

• alphanumeric, that is, letters, numbers, words, abbreviations

• figures, pictures, pictorials, icons, symbols, in various levels of abstraction, such as the outline of an airplane against the horizon

• shades of black, white or gray

• colours.

Unfortunately, many electronically generated displays have been fuzzy, often overly complex and colourful, hard to read, and required exact focusing and close attention, which may distract from the main task, for example, driving a car. In these cases the first three of the four “cardinal rules” listed above were often violated. Furthermore, many electronically generated pointers, markings and alphanumerics did not comply with established ergonomic design guidelines, especially when generated by line segments, scan lines or dot matrices. Although some of these defective designs were tolerated by the users, rapid innovation and improving display techniques allows many better solutions. However, the same rapid development leads to the fact that printed statements (even if current and comprehensive when they appear) are becoming obsolete quickly. Therefore, none are given in this text. Compilations have been published by Cushman and Rosenberg (1991), Kinney and Huey (1990), and Woodson, Tillman and Tillman (1991).

The overall quality of electronic displays is often wanting. One measure used to assess the image quality is the modulation transfer function (MTF) (Snyder 1985b). It describes the resolution of the display using a special sine-wave test signal; yet, readers have many criteria regarding the preference of displays (Dillon 1992).

Monochrome displays have only one colour, usually either green, yellow, amber, orange or white (achromatic). If several colours appear on the same chromatic display, they should be easily discriminated. It is best to display not more than three or four colours simultaneously (with preference being given to red, green, yellow or orange, and cyan or purple). All should strongly contrast with the background. In fact, a suitable rule is to design first by contrast, that is, in terms of black and white, and then to add colours sparingly.

In spite of the many variables that, singly and interacting with each other, affect the use of complex colour display, Cushman and Rosenberg (1991) compiled guidelines for use of colour in displays; these are listed in figure 7.

Figure 7. Guidelines for use of colours in displays

Other suggestions are as follows:

• Blue (preferably desaturated) is a good colour for backgrounds and large shapes. However, blue should not be used for text, thin lines or small shapes.

• The colour of alphanumeric characters should contrast with that of the background.

• When using colour, use shape as a redundant cue (e.g., all yellow symbols are triangles, all green symbols are circles, all red symbols are squares). Redundant coding makes the display much more acceptable for users who have colour-vision deficiencies.

• As the number of colours is increased, the sizes of the colour-coded objects should also be increased.

• Red and green should not be used for small symbols and small shapes in peripheral areas of large displays.

• Using opponent colours (red and green, yellow and blue) adjacent to one another or in an object/background relationship is sometimes beneficial and sometimes detrimental. No general guidelines can be given; a solution should be determined for each case.

• Avoid displaying several highly saturated, spectrally extreme colours at the same time.

Panels of Controls and Displays

Displays as well as controls should be arranged in panels so they are in front of the operator, that is, close to the person’s medial plane. As discussed earlier, controls should be near elbow height, and displays below or at eye height, whether the operator is sitting or standing. Infrequently operated controls, or less important displays, can be located further to the sides, or higher.

Often, information on the result of control operation is displayed on an instrument. In this case, the display should be located close to the control so that the control setting can be done without error, quickly and conveniently. The assignment is usually clearest when the control is directly below or to the right of the display. Care must be taken that the hand does not cover the display when operating the control.

Popular expectancies of control-display relations exist, but they are often learned, they may depend on the user’s cultural background and experience, and these relationships are often not strong. Expected movement relationships are influenced by the type of control and display. When both are either linear or rotary, the stereotypical expectation is that they move in corresponding directions, such as both up or both clockwise. When the movements are incongruent, in general the following rules apply:

• Clockwise for increase. Turning the control clockwise causes an increase in the displayed value.

• Warrick’s gear-slide rule. A display (pointer) is expected to move in the same direction as does the side of the control close to (i.e., geared with) the display.

The ratio of control and display displacement (C/D ratio or D/C gain) describes how much a control must be moved to adjust a display. If much control movement produces only a small display motion, once speaks of a high C/D ratio, and of the control as having low sensitivity. Often, two distinct movements are involved in making a setting: first a fast primary (“slewing”) motion to an approximate location, then a fine adjustment to the exact setting. In some cases, one takes as the optimal C/D ratio that which minimizes the sum of these two movements. However, the most suitable ratio depends on the given circumstances; it must be determined for each application.

Labels and Warnings

Labels

Ideally, no label should be required on equipment or on a control to explain its use. Often, however, it is necessary to use labels so that one may locate, identify, read or manipulate controls, displays or other equipment items. Labelling must be done so that the information is provided accurately and rapidly. For this, the guidelines in table 4 apply.

Table 4. Guidelines for labels

Source: Modified from Kroemer, Kroemer and Kroemer-Elbert 1994
(reproduced by permission of Prentice-Hall; all rights reserved).

Font (typeface) should be simple, bold and vertical, such as Futura, Helvetica, Namel, Tempo and Vega. Note that most electronically generated fonts (formed by LED, LCD or dot matrix) are generally inferior to printed fonts; thus, special attention must be paid to making these as legible as possible.

• The height of characters depends on the viewing distance:

viewing distance 35 cm, suggested height 22 mm

viewing distance 70 cm, suggested height 50 mm

viewing distance 1 m, suggested height 70 mm

viewing distance 1.5 m, suggested height at least 1 cm.

• The ratio of strokewidth to character height should be between 1:8 to 1:6 for black letters on white background, and 1:10 to 1:8 for white letters on black background.

• The ratio of character width to character height should be about 3:5.

• The space between letters should be at least one stroke width.

• The space between words should be at least one character width.

• For continuous text, mix upper- and lower-case letters; for labels, use upper-case letters only.

Warnings

Ideally, all devices should be safe to use. In reality, often this cannot be achieved through design. In this case, one must warn users of the dangers associated with product use and provide instructions for safe use to prevent injury or damage.

It is preferable to have an “active” warning, usually consisting of a sensor that notices inappropriate use, combined with an alerting device that warns the human of an impending danger. Yet, in most cases, “passive” warnings are used, usually consisting of a label attached to the product and of instructions for safe use in the user manual. Such passive warnings rely completely on the human user to recognize an existing or potential dangerous situation, to remember the warning, and to behave prudently.

Labels and signs for passive warnings must be carefully designed by following the most recent government laws and regulations, national and international standards, and the best applicable human engineering information. Warning labels and placards may contain text, graphics, and pictures—often graphics with redundant text. Graphics, particularly pictures and pictograms, can be used by persons with different cultural and language backgrounds, if these depictions are selected carefully. However, users with different ages, experiences, and ethnic and educational backgrounds, may have rather different perceptions of dangers and warnings. Therefore, design of a safe product is much preferable to applying warnings to an inferior product.

Back

In designing equipment it is of the utmost importance to take full account of the fact that a human operator has both capabilities and limitations in processing information, which are of a varying nature and which are found on various levels. Performance in actual work conditions strongly depends on the extent to which a design has either attended to or ignored these potentials and their limits. In the following a brief sketch will be offered of some of the chief issues. Reference will be made to other contributions of this volume, where an issue will be discussed in greater detail.

It is common to distinguish three main levels in the analysis of human information processing, namely, the perceptual level, the decision level and the motor level. The perceptual level is subdivided into three further levels, relating to sensory processing, feature extraction and identification of the percept. On the decision level, the operator receives perceptual information and chooses a reaction to it which is finally programmed and actualized on the motor level. This describes only the information flow in the simplest case of a choice reaction. It is evident, though, that perceptual information may accumulate and be combined and diagnosed before eliciting an action. Again, there may arise a need for selecting information in view of perceptual overload. Finally, choosing an appropriate action becomes more of a problem when there are several options some of which may be more appropriate than others. In the present discussion, the emphasis will be on the perceptual and decisional factors of information processing.

Perceptual Capabilities and Limits

Sensory limits

The first category of processing limits is sensory. Their relevance to information processing is obvious since processing becomes less reliable as information approaches threshold limits. This may seem a fairly trivial statement, but nonetheless, sensory problems are not always clearly recognized in designs. For example, alphanumerical characters in sign posting systems should be sufficiently large to be legible at a distance consistent with the need for appropriate action. Legibility, in turn, depends not only on the absolute size of the alphanumericals but also on contrast and—in view of lateral inhibition—also on the total amount of information on the sign. In particular, in conditions of low visibility (e.g., rain or fog during driving or flying) legibility is a considerable problem requiring additional measures. More recently developed traffic signposts and road markers are usually well designed, but signposts near and within buildings are often illegible. Visual display units are another example in which sensory limits of size, contrast and amount of information play an important role. In the auditory domain some main sensory problems are related to understanding speech in noisy environments or in poor quality audio transmission systems.

Feature extraction

Provided sufficient sensory information, the next set of information processing issues relates to extracting features from the information presented. Most recent research has shown ample evidence that an analysis of features precedes the perception of meaningful wholes. Feature analysis is particularly useful in locating a special deviant object amidst many others. For instance, an essential value on a display containing many values may be represented by a single deviant colour or size, which feature then draws immediate attention or “pops out”. Theoretically, there is the common assumption of “feature maps” for different colours, sizes, forms and other physical features. The attention value of a feature depends on the difference in activation of the feature maps that belong to the same class, for example, colour. Thus, the activation of a feature map depends on the discriminability of the deviant features. This means that when there are a few instances of many colours on a screen, most colour feature maps are about equally activated, which has the effect that none of the colours pops out.

In the same way a single moving advertisement pops out, but this effect disappears altogether when there are several moving stimuli in the field of view. The principle of the different activation of feature maps is also applied when aligning pointers that indicate ideal parameter values. A deviation of a pointer is indicated by a deviant slope which is rapidly detected. If this is impossible to realize, a dangerous deviation might be indicated by a change in colour. Thus, the general rule for design is to use only a very few deviant features on a screen and to reserve them only for the most essential information. Searching for relevant information becomes cumbersome in the case of conjunctions of features. For example, it is hard to locate a large red object amidst small red objects and large and small green objects. If possible, conjunctions should be avoided when trying to design for efficient search.

Separable versus integral dimensions

Features are separable when they can be changed without affecting the perception of other features of an object. Line lengths of histograms are a case in point. On the other hand, integral features refer to features which, when changed, change the total appearance of the object. For instance, one cannot change features of the mouth in a schematic drawing of a face without altering the total appearance of the picture. Again, colour and brightness are integral in the sense that one cannot change a colour without altering the brightness impression at the same time. The principles of separable and integral features, and of emergent properties evolving from changes of single features of an object, are applied in so-called integrated or diagnostic displays. The rationale of these displays is that, rather than displaying individual parameters, different parameters are integrated into a single display, the total composition of which indicates what may be actually wrong with a system.

Data presentation in control rooms is still often dominated by the philosophy that each individual measure should have its own indicator. Piecemeal presentation of the measures means that the operator has the task of integrating the evidence from the various individual displays so as to diagnose a potential problem. At the time of the problems in the Three Mile Island nuclear power plant in the United States some forty to fifty displays were registering some form of disorder. Thus, the operator had the task of diagnosing what was actually wrong by integrating the information from that myriad of displays. Integral displays may be helpful in diagnosing the kind of error, since they combine various measures into a single pattern. Different patterns of the integrated display, then, may be diagnostic with regard to specific errors.

A classical example of a diagnostic display, which has been proposed for nuclear control rooms, is shown in figure 1. It displays a number of measures as spokes of equal length so that a regular polygon always represents normal conditions, while different distortions may be connected with different types of problems in the process.

Figure 1. In the normal situation all parameter values are equal, creating a hexagon. In the deviation, some of the values have changed creating a specific distortion.

Not all integral displays are equally discriminable. To illustrate the issue, a positive correlation between the two dimensions of a rectangle creates differences in surface, while maintaining an equal shape. Alternatively, a negative correlation creates differences in shape while maintaining an equal surface. The case in which variation of integral dimensions creates a new shape has been referred to as revealing an emergent property of the patterning, which adds to the operator’s ability to discriminate the patterns. Emergent properties depend upon the identity and arrangement of parts but are not identifiable with any single part.

Object and configural displays are not always beneficial. The very fact that they are integral means that the characteristics of the individual variables are harder to perceive. The point is that, by definition, integral dimensions are mutually dependent, thus clouding their individual constituents. There may be circumstances in which this is unacceptable, while one may still wish to profit from the diagnostic patternlike properties, which are typical for the object display. One compromise might be a traditional bar graph display. On the one hand, bar graphs are quite separable. Yet, when positioned in sufficiently close vicinity, the differential lengths of the bars may together constitute an object-like pattern which may well serve a diagnostic aim.

Some diagnostic displays are better than others. Their quality depends on the extent that the display corresponds to the mental model of the task. For example, fault diagnosis on the basis of distortions of a regular polygon, as in figure 1, may still bear little relationship to the domain semantics or to the concept of the operator of the processes in a power plant. Thus, various types of deviations of the polygon do not obviously refer to a specific problem in the plant. Therefore, the design of the most suitable configural display is one that corresponds to the specific mental model of the task. Thus it should be emphasized that the surface of a rectangle is only a useful object display when the product of length and width is the variable of interest!

Interesting object displays stem from three-dimensional representations. For instance, a three-dimensional representation of air traffic—rather than the traditional two-dimensional radar representation—may provide the pilot with a greater “situational awareness” of other traffic. The three-dimensional display has been shown to be much superior to a two-dimensional one since its symbols indicate whether another aircraft is above or below one’s own.

Degraded conditions

Degraded viewing occurs under a variety of conditions. For some purposes, as with camouflage, objects are intentionally degraded so as to prevent their identification. On other occasions, for example in brightness amplification, features may become too blurred to allow one to identify the object. One research issue has concerned the minimal number of “lines” required on a screen or “the amount of detail” needed in order to avoid degradation. Unfortunately, this approach to image quality has not led to unequivocal results. The problem is that identifying degraded stimuli (e.g., a camouflaged armoured vehicle) depends too much on the presence or absence of minor object-specific details. The consequence is that no general prescription about line density can be formulated, except for the trivial statement that degradation decreases as the density increases.

Features of alphanumeric symbols

A major issue in the process of feature extraction concerns the actual number of features which together define a stimulus. Thus, the legibility of ornate characters like Gothic letters is poor because of the many redundant curves. In order to avoid confusion, the difference between letters with very similar features—like the i and the l, and the c and the e—should be accentuated. For the same reason, it is recommended to make the stroke and tail length of ascenders and descenders at least 40% of the total letter height.

It is evident that discrimination among letters is mainly determined by the number of features which they do not share. These mainly consist of straight line and circular segments which may have horizontal, vertical and oblique orientation and which may differ in size, as in lower- and upper-case letters.

It is obvious that, even when alphanumericals are well discriminable, they may easily lose that property in combination with other items. Thus, the digits 4 and 7 share only a few features but they do not do well in the context of larger otherwise identical groups (e.g., 384 versus 387) There is unanimous evidence that reading text in lower case is faster than in capitals. This is usually ascribed to the fact that lower case letters have more distinct features (e.g., dog, cat versus DOG, CAT). The superiority of lower case letters has not only been established for reading text but also for road signs such as those used for indicating towns at the exits of motorways.

Identification

The final perceptual process is concerned with identification and interpretation of percepts. Human limits arising on this level are usually related to discrimination and finding the appropriate interpretation of the percept. The applications of research on visual discrimination are manifold, relating to alphanumerical patterns as well as to more general stimulus identification. The design of brake lights in cars will serve as an example of the last category. Rear-end accidents account for a considerable proportion of traffic accidents, and are due in part to the fact that the traditional location of the brake light next to the rear lights makes it poorly discriminable and therefore extends the driver’s reaction time. As an alternative, a single light has been developed which appears to reduce the accident rate. It is mounted in the centre of the rear window at approximately eye level. In experimental studies on the road, the effect of the central braking light appears to be less when subjects are aware of the aim of the study, suggesting that stimulus identification in the traditional configuration improves when subjects focus on the task. Despite the positive effect of the isolated brake light, its identification might still be further improved by making the brake light more meaningful, giving it the form of an exclamation mark, “!”, or even an icon.

Absolute judgement

Very strict and often counterintuitive performance limits arise in cases of absolute judgement of physical dimensions. Examples occur in connection with colour coding of objects and the use of tones in auditory call systems. The point is that relative judgement is far superior to absolute judgement. The problem with absolute judgement is that the code has to be translated into another category. Thus a specific colour may be linked with an electrical resistance value or a specific tone may be intended for a person for which an ensuing message is meant. In fact, therefore, the problem is not one of perceptual identification but rather of response choice, which will be discussed later in this article. At this point it suffices to remark that one should not use more than four or five colours or pitches so as to avoid errors. When more alternatives are needed one may add extra dimensions, like loudness, duration and components of tones.

Word reading

The relevance of reading separate word units in traditional print is demonstrated by various widely experienced evidence, such as the fact that reading is very much hampered when spaces are omitted, printing errors remain often undetected, and it is very hard to read words in alternating cases (e.g., ALTeRnAtInG). Some investigators have emphasized the role of word shape in reading word units and suggested that spatial frequency analysers may be relevant in identifying word shape. In this view meaning would be derived from total word shape rather than by letter-by-letter analysis. Yet, the contribution of word shape analysis is probably limited to small common words—articles and endings—which is consistent with the finding that printing errors in small words and endings have a relatively low probability of detection.

Text in lower case has an advantage over upper case which is due to a loss of features in the upper case. Yet, the advantage of lower case words is absent or may even be reversed when searching for a single word. It could be that factors of letter size and letter case are confounded in searching: Larger-sized letters are detected more rapidly, which may offset the disadvantage of less distinctive features. Thus, a single word may be about equally legible in upper case as in lower case, while continuous text is read faster in lower case. Detecting a SINGLE capital word amidst many lower case words is very efficient, since it evokes pop-out. An even more efficient fast detection can be achieved by printing a single lower case word in bold, in which case the advantages of pop-out and of more distinctive features are combined.

The role of encoding features in reading is also clear from the impaired legibility of older low-resolution visual display unit screens, which consisted of fairly rough dot matrices and could portray alphanumericals only as straight lines. The common finding was that reading text or searching from a low-resolution monitor was considerably slower than from a paper-printed copy. The problem has largely disappeared with the present-day higher-resolution screens. Besides letter form there are a number of additional differences between reading from paper and reading from a screen. The spacing of the lines, the size of the characters, the type face, the contrast ratio between characters and background, the viewing distance, the amount of flicker and the fact that changing pages on a screen is done by scrolling are some examples. The common finding that reading is slower from computer screens—although comprehension seems about equal—may be due to some combination of these factors. Present-day text processors usually offer a variety of options in font, size, colour, format and style; such choices could give the false impression that personal taste is the major reason.

Icons versus words

In some studies the time taken by a subject in naming a printed word was found to be faster than that for a corresponding icon, while both times were about equally fast in other studies. It has been suggested that words are read faster than icons since they are less ambiguous. Even a fairly simple icon, like a house, may still elicit different responses among subjects, resulting in response conflict and, hence, a decrease in reaction speed. If response conflict is avoided by using really unambiguous icons the difference in response speed is likely to disappear. It is interesting to note that as traffic signs, icons are usually much superior to words, even in the case where the issue of understanding language is not seen as a problem. This paradox may be due to the fact that the legibility of traffic signs is largely a matter of the distance at which a sign can be identified. If properly designed, this distance is larger for symbols than for words, since pictures can provide considerably larger differences in shape and contain less fine details than words. The advantage of pictures, then, arises from the fact that discrimination of letters requires some ten to twelve minutes of arc and that feature detection is the initial prerequisite for discrimination. At the same time it is clear that the superiority of symbols is only guaranteed when (1) they do indeed contain little detail, (2) they are sufficiently distinct in shape and (3) they are unambiguous.

Capabilities and Limits for Decision

Once a precept has been identified and interpreted it may call for an action. In this context the discussion will be limited to deterministic stimulus-response relations, or, in other words, to conditions in which each stimulus has its own fixed response. In that case the major problems for equipment design arise from issues of compatibility, that is, the extent to which the identified stimulus and its related response have a “natural” or well-practised relationship. There are conditions in which an optimal relation is intentionally aborted, as in the case of abbreviations. Usually a contraction like abrvtin is much worse than a truncation like abbrev. Theoretically, this is due to the increasing redundancy of successive letters in a word, which allows “filling out” final letters on the basis of earlier ones; a truncated word can profit from this principle while a contracted one cannot.

Mental models and compatibility

In most compatibility problems there are stereotypical responses derived from generalized mental models. Choosing the null position in a circular display is a case in point. The 12 o’clock and 9 o’clock positions appear to be corrected faster than the 6 o’clock and 3 o’clock positions. The reason may be that a clockwise deviation and a movement in the upper part in the display are experienced as “increases” requiring a response that reduces the value. In the 3 and 6 o’clock positions both principles conflict and they may therefore be handled less efficiently. A similar stereotype is found in locking or opening the rear door of a car. Most people act on the stereotype that locking requires a clockwise movement. If the lock is designed in the opposite way, continuous errors and frustration in trying to lock the door are the most likely result.

With respect to control movements the well-known Warrick’s principle on compatibility describes the relation between the location of a control knob and the direction of the movement on a display. If the control knob is located to the right of the display, a clockwise movement is supposed to move the scale marker up. Or consider moving window displays. According to most people’s mental model, the upward direction of a moving display suggests that the values go up in the same way in which a rising temperature in a thermometer is indicated by a higher mercury column. There are problems in implementing this principle with a “fixed pointer-moving scale” indicator. When the scale in such an indicator moves down, its value is intended to increasing. Thus a conflict with the common stereotype occurs. If the values are inverted, the low values are on the top of the scale, which is also contrary to most stereotypes.

The term proximity compatibility refers to the correspondence of symbolic representations to people’s mental models of functional or even spatial relationships within a system. Issues of proximity compatibility are more pressing as the mental model of a situation is more primitive, global or distorted. Thus, a flow diagram of a complex automated industrial process is often displayed on the basis of a technical model which may not correspond at all with the mental model of the process. In particular, when the mental model of a process is incomplete or distorted, a technical representation of the progress adds little to develop or correct it. A daily-life example of poor proximity compatibility is an architectural map of a building that is intended for viewer orientation or for showing fire escape routes. These maps are usually entirely inadequate—full of irrelevant details—in particular for people who have only a global mental model of the building. Such convergence between map reading and orientation comes close to what has been called “situational awareness”, which is particularly relevant in three-dimensional space during an air flight. There have been interesting recent developments in three-dimensional object displays, representing attempts to achieve optimal proximity compatibility in this domain.

Stimulus-response compatibility

An example of stimulus-response (S-R) compatibility is typically found in the case of most text processing programs, which assume that operators know how commands correspond to specific key combinations. The problem is that a command and its corresponding key combination usually fail to have any pre-existing relation, which means that the S-R relations must be learned by a painstaking process of paired-associate learning. The result is that, even after the skill has been acquired, the task remains error-prone. The internal model of the program remains incomplete since less practised operations are liable to be forgotten, so that the operator can simply not come up with the appropriate response. Also, the text produced on the screen usually does not correspond in all respects to what finally appears on the printed page, which is another example of inferior proximity compatibility. Only a few programs utilize a stereotypical spatial internal model in connection with stimulus-response relations for controlling commands.

It has been correctly argued that there are much better pre-existing relations between spatial stimuli and manual responses—like the relation between a pointing response and a spatial location, or like that between verbal stimuli and vocal responses. There is ample evidence that spatial and verbal representations are relatively separate cognitive categories with little mutual interference but also with little mutual correspondence. Hence, a spatial task, like formatting a text, is most easily performed by spatial mouse-type movement, thus leaving the keyboard for verbal commands.

This does not mean that the keyboard is ideal for carrying out verbal commands. Typing remains a matter of manually operating arbitrary spatial locations which are basically incompatible with processing letters. It is actually another example of a highly incompatible task which is only mastered by extensive practise, and the skill is easily lost without continuous practice. A similar argument can be made for shorthand writing, which also consists of connecting arbitrary written symbols to verbal stimuli. An interesting example of an alternative method of keyboard operation is a chording keyboard.

The operator handles two keyboards (one for the left and one for the right hand) both consisting of six keys. Each letter of the alphabet corresponds to a chording response, that is, a combination of keys. The results of studies on such a keyboard showed striking savings in the time needed for acquiring typing skills. Motor limitations limited the maximal speed of the chording technique but, still, once learned, operator performance approached the speed of the conventional technique quite closely.

A classical example of a spatial compatibility effect concerns the traditional arrangements of stove burner controls: four burners in a 2 ´ 2 matrix, with the controls in a horizontal row. In this configuration, the relations between burner and control are not obvious and are poorly learned. However, despite many errors, the problem of lighting the stove, given time, can usually be solved. The situation is worse when one is faced with undefined display-control relations. Other examples of poor S-R compatibility are found in the display-control relations of video cameras, video recorders and television sets. The effect is that many options are never used or must be studied anew at each new trial. The claim that “it is all explained in the manual”, while true, is not useful since, in practice, most manuals are incomprehensible to the average user, in particular when they attempt to describe actions using incompatible verbal terms.

Stimulus-stimulus (S-S) and response-response (R-R) compatibility

Originally S-S and R-R compatibility were distinguished from S-R compatibility. A classical illustration of S-S compatibility concerns attempts in the late forties to support auditory sonar by a visual display in an effort to enhance signal detection. One solution was sought in a horizontal light beam with vertical perturbations travelling from left to right and reflecting a visual translation of the auditory background noise and potential signal. A signal consisted of a slightly larger vertical perturbation. The experiments showed that a combination of the auditory and visual displays did not do better than the single auditory display. The reason was sought in a poor S-S compatibility: the auditory signal is perceived as a loudness change; hence visual support should correspond most when provided in the form of a brightness change, since that is the compatible visual analogue of a loudness change.

It is of interest that the degree of S-S compatibility corresponds directly to how skilled subjects are in cross-modality matching. In a cross-modality match, subjects may be asked to indicate which auditory loudness corresponds to a certain brightness or to a certain weight; this approach has been popular in research on scaling sensory dimensions, since it allows one to avoid mapping sensory stimuli to numerals. R-R compatibility refers to correspondence of simultaneous and also of successive movements. Some movements are more easily coordinated than others, which provides clear constraints for the way a succession of actions—for example, successive operation of controls—is most efficiently done.

The above examples show clearly how compatibility issues pervade all user-machine interfaces. The problem is that the effects of poor compatibility are often softened by extended practice and so may remain unnoticed or underestimated. Yet, even when incompatible display-control relations are well-practised and do not seem to affect performance, there remains the point of a larger error probability. The incorrect compatible response remains a competitor for the correct incompatible one and is likely to come through on occasion, with the obvious risk of an accident. In addition, the amount of practice required for mastering incompatible S-R relations is formidable and a waste of time.

Limits of Motor Programming and Execution

One limit in motor programming was already briefly touched upon in the remarks on R-R compatibility. The human operator has clear problems in carrying out incongruent movement sequences, and in particular, changing from the one to another incongruent sequence is hard to accomplish. The results of studies on motor coordination are relevant to the design of controls in which both hands are active. Yet, practice can overcome much in this regard, as is clear from the surprising levels of acrobatic skills.

Many common principles in the design of controls derive from motor programming. They include the incorporation of resistance in a control and the provision of feedback indicating that it has been properly operated. A preparatory motor state is a highly relevant determinant of reaction time. Reacting to an unexpected sudden stimulus may take an additional second or so, which is considerable when a fast reaction is needed—as in reacting to a lead car’s brake light. Unprepared reactions are probably a main cause of chain collisions. Early warning signals are beneficial in preventing such collisions. A major application of research on movement execution concerns Fitt’s law, which relates movement, distance and the size of the target that is aimed at. This law appears to be quite general, applying equally to an operating lever, a joystick, a mouse or a light pen. Among others, it has been applied to estimate the time needed to make corrections on computer screens.

There is obviously much more to say than the above sketchy remarks. For instance, the discussion has been almost fully limited to issues of information flow on the level of a simple choice reaction. Issues beyond choice reactions have not been touched upon, nor problems of feedback and feed forward in the ongoing monitoring of information and motor activity. Many of the issues mentioned bear a strong relation to problems of memory and of planning of behaviour, which have not been addressed either. More extensive discussions are found in Wickens (1992), for example.

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