HUMAN-COMPUTER INTERACTION SECOND EDITION
Dix, Finlay, Abowd and Beale

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Chapter 4 Usability paradigms and principles Synthesizability Page 164

When an operation changes some aspect of the internal state, it is important that the change is seen by the user. The principle of honesty relates to the ability of the user interface to provide an observable and informative account of such change. In the best of circumstances, this notification can come immediately, requiring no further interaction initiated by the user. Or at the very least, the notification should appear eventually, after explicit user directives to make the change observable. A good example of the distinction between immediacy and eventuality can be seen in the comparison between command language interfaces and visual desktop interfaces for a file management system. You have moved a file from one directory to another. The principle of honesty implies that after moving the file to its new location in the file system you are then able to determine its new whereabouts. In a command language system, you would typically have to remember the destination directory and then ask to see the contents of that directory in order to verify that the file has been moved (in fact, you would also have to check that the file is no longer in its original directory to determine that it has been moved and not copied). In a visual desktop interface, a visual representation (or icon) of the file to be moved is dragged from its containing directory and placed in its destination directory where it remains visible (assuming the destination folder is selected to reveal its contents). In this case, the user need not expend any more effort to assess the result of the move operation. In this case, the visual desktop is immediately honest.

Chapter 4 Usability paradigms and principles Familiarity Page 166

Some psychologists argue that there are intrinsic properties, or affordances, of any visual object that suggest to us how they can be manipulated. The appearance of the object stimulates a familiarity with its behaviour. For example, the shape of a door handle can suggest how it should be manipulated to open a door, and a key on a keyboard suggests to us that it can be pushed. In the design of a graphical user interface, it is implied that a soft button used in a form's interface suggests it should be pushed (though it does not suggest how it is to be pushed via the mouse). Effective use of the affordances which exist for interface objects can enhance the familiarity of the interactive system.

Chapter 4 Usability paradigms and principles Consistency Page 166

Consistency relates to the likeness in behaviour arising from similar situations or similar task objectives. Consistency is probably the most widely mentioned principle in the literature on user interface design. 'Be consistent!' we are constantly urged. The user relies on a consistent interface. However, the difficulty of dealing with consistency is that it can take many forms. Consistency is not a single property of an interactive system that is either satisfied or not satisfied. Instead, consistency must be applied relative to something. Thus we have consistency in command naming, or consistency in command/argument invocation.

Chapter 4 Usability paradigms and principles Substitutivity Page 170

Equal opportunity blurs the distinction between input and output at the interface. The user has the choice of what is input and what is output; in addition, output can be reused as input. Thimbleby describes this principle as, 'If you can see it, you can use it!' It is a common belief that input and output are separate. Many have stressed the significance of the link between input and output. Equal opportunity pushes that view to the extreme. For example, in spreadsheet programs, the user fills in some cells and the system automatically determines the values attributed to some other cells. Conversely, if the user enters values for those other cells, the system would compute the values for the first ones. In this example, it is not clear which cells are the inputs and which are the outputs. Furthermore, this distinction might not be clear or useful to the user. In a drawing package, the user may draw a line by direct manipulation and the system would compute the length of the line; or conversely, the user may specify the line coordinates and the system would draw the line. Both means of manipulating the line are equally important and must be made equally available. Note that equal opportunity implies that the system is not pre-emptive towards the user.

Chapter 4 Usability paradigms and principles Customizability Page 171

Customizability is the modifiability of the user interface by the user or the system. From the system side, we are not concerned with modifications that would be attended to by a programmer actually changing the system and its interface during system maintenance. Rather, we are concerned with the automatic modification that the system would make based on its knowledge of the user. We distinguish between the user-initiated and system-initiated modification, referring to the former as adaptability and the latter as adaptivity.

Chapter 4 Usability paradigms and principles Customizability Page 171

Adaptability refers to the user's ability to adjust the form of input and output. This customization could be very limited, with the user only allowed to adjust the position of soft buttons on the screen or redefine command names. This type of modifiability, which is restricted to the surface of the interface, is referred to as lexical customization. The overall structure of the interaction is kept unchanged. The power given to the user can be increased by allowing the definition of macros to speed up the articulation of certain common tasks. In the extreme, the interface can provide the user with programming language capabilities, such as the UNIX shell or the script language Hypertalk in HyperCard. In these cases, Thimbleby points out that it would be suitable to apply well-known principles of programming languages to the user's interface programming language.

Chapter 4 Usability paradigms and principles Customizability Page 171

Adaptivity is automatic customization of the user interface by the system. Decisions for adaptation can be based on user expertise or observed repetition of certain task sequences. The distinction between adaptivity and adaptability is that the user plays an explicit role in adaptability, whereas his role in an adaptive interface is more implicit. A system can be trained to recognize the behaviour of an expert or novice and accordingly adjust its dialog control or help system automatically to match the needs of the current user. This is in contrast with a system which would require the user to classify himself as novice or expert at the beginning of a session. We discuss adaptive systems further in Chapter 12. Automatic macro construction is a form of programming by example, combining adaptability with adaptivity in a simple and useful way. Repetitive tasks can be detected by observing user behaviour and macros can be automatically (or with user consent) constructed from this observation to perform repetitive tasks automatically.

Chapter 4 Usability paradigms and principles Observability Page 172

Observability allows the user to evaluate the internal state of the system by means of its perceivable representation at the interface. As we described in Chapter 3, evaluation allows the user to compare the current observed state with his intention within the task--action plan, possibly leading to a plan revision. Observability can be discussed through five other principles: browsability, defaults, reachability, persistence and operation visibility. Operation visibility was covered in Section 4.3.1 in relation to predictability. The remaining four are discussed next.

Chapter 4 Usability paradigms and principles Observability Page 172

Browsability allows the user to explore the current internal state of the system via the limited view provided at the interface. Usually the complexity of the domain does not allow the interface to show all of the relevant domain concepts at once. Indeed, this is one reason why the notion of task is used, in order to constrain the domain information needed at one time to a subset connected with the user's current activity. While you may not be able to view an entire document's contents, if you are only interested in its overall structure, you may be able to see all of an outline view of the document. Even with a restriction of concepts relevant to the current task, it is probable that all of the information a user needs to continue work on that task is not immediately perceivable. Or perhaps the user is engaged in a multi-threaded dialog covering several tasks. There needs to be a way for the user to investigate, or browse, the internal state. This browsing itself should not have any side-effects on that state; that is, the browsing commands should be passive with respect to the domain-specific parts of the internal state.

Chapter 4 Usability paradigms and principles Task conformance Page 175

Discussion of task conformance has its roots in an attempt to understand the success of direct manipulation interfaces. We can view the direct manipulation interface as a separate world from that inside the system. Task completeness covers only one part of the conformance. This separate world is understood and operated upon by the user. With the intuition of the Hutchins, Hollan and Norman model-world metaphor mentioned earlier we require that the task, as represented by the world of the interface, matches the task as understood by the user and supported by the system. If the model-world metaphor satisfies the principle of task adequacy, then the user will be directly on his task plan, minimizing the effort required in the articulation and observation translations discussed in the interaction framework of Chapter 3.

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