Stuart K. Card, Thomas P. Moran, & Allen Newell
This essay addresses the potential contribution of an applied psychology to developing Xerox's Office Information Systems. Three years ago, under the impact of a cognitive psychology that finally seemed to be developing an adequate theoretical base, the Applied Information Processing Psychology Project (AlP for short) started down the path to creating a new applied psychology that would make use of that base. This essay is a restatement of the original vision in the light of our attempts in the interval since then.
The Problem and the Vision
Within the world of Xerox Office Information Systems, some propositions are almost self-evident:
A. The performance of an office system depends strongly on its being easy to learn and use.
B. Because the users of office systems are not technically oriented, the importance of the interface between an office system and its users is magnified.
C. The marketability of an office system depends on the ease, grace, and apparent simplicity of its use, even beyond its actual performance once firmly incorporated into a customer's operation.
D. The style of information systems most favored by Xerox - - namely, highly interactive, decentralized, personal computing - - is exactly the one that most emphasizes the human- system interface.
These propositions accurately reflect the large potential contribution to be made through good design of the human- system interface. Further argument on this point seems unnecessary.
This does not establish automatically that an applied psychology is needed for good
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interface design. Interface design is still largely an art. Sensitive designers often produce elegant interfaces, and the iteration of designs in response to user reactions can remove rough edges. But an applied science of how humans use office systems --how they think, plan, follow procedures, perceive, communicate - - should have much to say about the design of the interface. That any room for doubt exists about an applied psychology's contribution reflects the variability and complexity of human behavior. 'It also reflects psychology's still immature state compared with the physical (and more recently the biological) sciences. But all sciences grow, and psychology's potential for substantial contribution to an applied area, such as office information systems, must be continually reassessed.
There has indeed been growth in psychology, especially in cognitive psychology, which has come to view man as an active processor of information who can represent his environment symbolically, create goals to express his desires, and engage in strategies of action to attain these goals. The key to treating the information processing aspects of man scientifically and quantitatively has been the developing understanding of man- made information processing systems, i.e., computer and control systems.
This cognitive psychology provides us with a vision of an applied psychology of office systems that is mundane by the standards of physical engineering and science, but novel in the extreme for psychology. It almost catches the breath:
1. Theoretical base: There will be a unified model of the office person as he interacts with office information systems; this framework will allow the progressive cumulation of both facts and theory.
2. Calculations: The theory will permit calculations of results, especially of the "back- of- the- envelope" type so dear to the practiCing engineer and designer; better approximations may be had by applying more effort.
3. Scope of application: Contributions will occur across the full range of the design process, from the evaluation of office systems, both prototypes and proposals, to the generation of new design conceptions.
4. Integration into design practice: The results will become tools of the working designer, not just the professional psychologist, thus maximizing the opportunities for application.
This is a vision, not a present reality. Visions are useful if they are close enough to reality for current finite efforts to make perceptible headway toward them. Enough ingredients and demonstrations of power have been provided by recent advances in psychology to create a suitable base, though much in this vision is still foreign to traditional techniques (back- of- the- envelope calculations, for example) and must be
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forged on site. Still, it was this VISion, and the assessment of its attainability, that underlay the initiation of AlP at PARC three years ago. It is the perceptible headway that we've made in the three years that leads us now to a restatement.
The next section will make concrete what we mean by an applied psychology that is quantitative and admits of calculation, yet deals in goals and symbolic behavior. Then we will project some of the ways that we see such an applied psychology operating when it is mature. This will lead to a brief assessment of where AlP currently stands in creating that mature science, and what needs to be done to move along that path.
The Shape of an Applied Psychology
The Model of the User
Central to the enterprise is a model of the human user and how he interacts with office systems. This model comes from a wide range of basic experiments in cognitive psychology that have gradually let us piece together a reasonable picture of the functional information processing architecture of the human.
The simplest version of the model takes the user to have a single long term memory of unlimited size which holds essentially permanent knowledge. There is also a working memory of very limited size which holds the knowledge of the current task context; this represents the user's focus of attention. What the user does at each given instant of time is determined by the contents of this working memory. Knowledge flows into the working memory from both the external environment of the user and from his long term memory. Both of these flows are determined or moderated by the knowledge in the working memory at each instant. Similarly, the user's initiation and control of external (motor) actions and of the addition of new knowledge to his long term memory is determined by the contents of working memory.
Some important general limitations of the user of office systems can be derived directly from this simple view of the human information processing architecture:
(1) The user can do only one main thing at a time - - what the working memory determines at each instant.
(2) He can work only so fast - - there is an upper bound to the cycle time of the system (in the range of 10 to 20 cycles per second).
(3) His working memory is limited (less than a dozen items of knowledge) and overloading the working memory causes some immediate knowledge to be lost.
(4) His knowledge in long term memory can only be retrieved if it can be accessed via the knowledge already in working memory. Thus, complete failures to recall relevant knowledge are possible, as are retrievals of inappropriate knowledge because, the retrieval clues were inadequate.
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Calculating from Theory: Goals and Methods
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To be able to calculate with a theory, one must be able to represent the behavior of the system in terms of some primitive components and how they are composed. Editing manuscripts with interactive computer editing systems provides a good example. It is typical of the operations that will occur in future office systems. One important quantity operations are indicated by CAPITALS, and their average times (empirically determined) are given in the right column:
Applied Intonnation Processing Psychology systems are composed of similar command and argument specifications.
The use of such a formalism can be illustrated by an opportunity that fell our way. It guide the design process into considering alternative product schemes of quite different character. above and assuming a computer- formatting system with plausible properties. Enough contact could be made with the operations in the systems we have stUdied, so that numerical estimates of the duration of each unit task could be calculated. Taking these together with the frequencies of occurrence yielded a total estimated time to format a typical page. We also investigated the non- additivity of the unit tasks (since related tasks done together avoid repeating common operations) and showed that the correction was probably too small to make any difference.
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The estimate obtained is surely in error. However, it is probably substantially better than the more intuitive estimates the designers were drawing from their own experience. It cannot be checked directly, since page- formatting systems do not exist. We did create one quick analog in the laboratory and took measurements of actual performance to provide some gross check. We could not quite confine the analysis to the back of an envelope; it took about a week. Even so, that time scale is well matched to the demands of the actual design process it was attempting to aid.
Calculating from Theory: POinting to Text on a Display
Another example of a calculational applied psychology comes from the problem of how users point to text on computer displays. There are many different devices for moving the cursor from one point on the screen to another, and the usual range of opinion exists about the advantages and disadvantages of different devices. This is a classic human factors situation where the specific devices would be evaluated by running experiments under a range of conditions (some evaluations of cursor movement devices exist in the literature) .
One can do better than this, however. According to a law proposed by Paul Fitts in 1954, the time it takes a skilled user to move his arm in a task demanding accurate placement is given by
t = ko + k log2(dls + .5) ,
where d is the distance to be moved and s is the size of the target region into which the movement must be made. The smaller s is, the more accuracy is demanded.. The formula says that what counts is only the relative accuracy (dis). Further, the time increases only as the logarithm of this accuracy (the number of bits in the discrimination). The constants, ko and k, reflect the movement situation and the skill of the operator. Fitts' Law is one of the most robust results in psychology, and it has been verified in a wide range of experiments.
The usefulness of Fitts' Law can be seen by considering another question of current interest to SOD: how good is the "mouse" as a cursor control device, and are there any better alternative devices? Fitts' Law can be applied to the case of cursor movement devices by taking d to be the distance on the screen that the cursor has to move and taking s to be the size of the target text. Data from a series of experiments that we ran with the mouse give the following fit to Fitts' Law:
tmouse = 1.03 + .096 log2(dls + .5) seconds.
One more step is required to understand the implication of this result. In the literature on motor control, the value of .1 seconds per bit for the constant k shows up repeatedly in different arrangements. as a lower bound, i.e., as the fastest the human can operate. This appears to be a.limit due to the information processing necessary to guide the motion.
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The formula for the mouse above not only tells us how fast it is for different targets, but also it tells us that it is unlikely that other continuous- movement devices can be found that improve much upon it. (Even pointing directly to the screen with a finger would not be much faster!) What hope there is lies in trimming the one second set- up constant, ko' which involves the basic reaction needed to respond to the occurrence of the task itself, no matter what device is used, and the pushing of the mouse button. In fact, we have tested other pointing devices in our laboratory, and none are faster than the mouse.
The Yield of an Applied Psychology
The previous section attempted to give some flavor of the applied psychology based on viewing the user as an information processing system. Drawing on present work, we showed how one might calculate approximate answers to quantities that are of applied interest in office information systems. Here we would like to reach beyond what we now feel sure about to give some examples of ways we envision applications occurring.
Simulated Users
System simulation is a standard design technique in computer science. It is used to evaluate proposed designs before they actually exist and to permit explorations of design variations without the expense and time to construct alternative prototype systems. Simulations always work with some simulated model of the external environment. In some areas, such as standard computer time sharing systems, rather simple models of the environment are adequate. However, for the office information systems envisioned by Xerox, the interaction between the user and the systems is too intimate for any simple model to suffice.
A "Simulated User" is a program that will produce the behavior of a human user in conjunction with a simulation of a proposed system. It will generate sequences of tasks to do according to some probabilistic model of the task domain, and for each task it will generate commands to instruct the computer system to carry out these tasks. It will choose methods based on the same principles that appear to govern human choices, and it will make errors with the same relative frequencies. A Simulated User can be tuned to show certain kinds of user characteristics (e.g., to be a particularly sloppy user or a highly efficient one); and it can behave at any level of detail, depending on how it is to be used.
A Simulated User is a way of packaging psychological knowledge that makes it fit directly into the design tools of the system designer. Simply by using it in conjunction with his simulation he brings to bear relevant psychological effects in a direct and continuous way. Furthermore, the Simulated User is cast in exactly the terms that a practicing software or hardware designer can understand; he can explore its behavior when it turns up unexpected issues in the design. It will not be a black box.
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A Design Representation for Command Languages
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The artistic side of design lies largely in the generation of ideas and the scientific side lies largely in their testing. It is easier to imagine how to use scientific knowledge of the user to evaluate designs than to help in their generation. Yet a good applied psychology must do both.
Common to all interactive computer systems is the command language - - the language used by the user for specifying to the computer what he wants it to do. Their design is an important aspect of the design of interactive office systems. It is almost completely an intuitive art currently, with its inevitable collection of lore. One notion is the "User's Model": that the user develops a conceptual picture of the interactive system that serves to guide his actions and expectations of how the system will behave. Whether a system is simple and elegant or awkward and inconsistent is a reflection of its implicit User's Model. A simple User's Model reflects in ease of learning, though it is recognized that any finite system can eventually be learned to permit expert performance. Unfortunately, no scientific substance has ever been given to the notion of a User's Model, and it remains essentially a mythical beast.
Imagine, then, a notation for specifying command languages with two main components.
The first is a notation for specifying the computer system being controlled by the user via the command language. This specifies the user's view - - the entities that the user understands, their relations to each other, and the types of actions the user sees the computer as being able to perform. In short, this is the User's Model. The actual implementation of the system must stay within the limits imposed by the User's Model --or the user's view will actually be inconsistent - - but otherwise it is quite free.
The second component of the notation is a grammar for the command language itself, defined in terms of linguistic constructs .. commands, arguments, contexts, state variables - - and in terms of how it designates and evokes the elements of the User's Model. The user sees his commands as designating to the system various elements as defined in the User's Model. The grammar can specify command languages of arbitrary complexity and complexion; it encompasses all possibilities for command languages, given a User's Model.
By providing a separate representation for the User's Model and for the command language per se, this scheme permits the User's Model to be designed explicitly, rather than to provide only informal heuristic guidance. This wholly changes the space in which user- system interfaces are designed.
A command language representation embodying a User's Model will not look to the practicing designer like a psychological theory. He will see it simply as a symbolic tool that lets him write down aspects of proposed systems - - from sketches to completed specifications. Nevertheless, the representation will be a psychological theory.
Experiments will have verified that the representation reflects how humans actually view
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office systems. For example, users' learning of systems will be describable in terms of acquisition of the User's Model. Doing what comes naturally within the representation will lead to designs that take human cognitive structure into account.
The Style of an Applied Psychology
Questions concerning the user occur·, everywhere in the design of office information systems. Often they are not the central question, but operate more as a check against solutions being user- foolish. Being pervasive, these applications will be made by the designers themselves or they will not occur at all. An important component of our vision, then, is to get the tools into the hands and heads of the practicing designer. We do not thereby believe the designer, by training a software or hardware professional, is to become also a psychologist. Gaps in expertise always exists between disciplines. But the main result of an applied psychology will be measured by its lifting the whole design process in its consideration of the human interface, not by a few psychologically deep applications, made by the psychologists themselves (though we hope for those too).
The examples we have given already show some ways in which this transfer will occur.
The Simulated User and the command language representation (with the User's Model) both provide tools that are used directly by the designer and incorporate psychological knowledge in their very structure. More generally, the psychological theory we've illustrated is cast in the same language as that used by the engineer, so he can assimilate it easily.
Perhaps a small example will show what we mean. Recently we gave a seminar about the pointer studies and Fitts' Law. A few hours afterwards, Lynn Conway, the Manager of
Perhaps a small example will show what we mean. Recently we gave a seminar about the pointer studies and Fitts' Law. A few hours afterwards, Lynn Conway, the Manager of