Chapter 4

Usability Paradigms and Principles

4.1 What was the problem with the synthesis example comparing a command language interface with a visual interface? Can you suggest a fix to make a visual interface really immediately honest?

answer

To demonstrate the principle of synthesizability within learnability, it was stated in this example that a visual interface to a file management system provided immediate information about the changed location of some file after a move operation performed by the user. In contrast, a command language interface requires the user to remember the directory to which a file is moved and explicitly issue commands to browse the directory to verify that the file has been moved. To be really sure that a move occurred, the user would also have to browse the original directory to determine that the file was no longer there. The fallacy in this argument is the assumption that visual file management systems always provide information about the new whereabouts of a moved file. To take a counterexample using the Macintosh instance from the text, if a file is moved from an open folder (in which the contents of the folder are revealed to the user) to a closed folder (contents not revealed), then the location of the moved file is not indicated to the user unless she remembers to open up the destination folder to reveal its contents. This is an example of eventual honesty and not immediate as the example suggests.

We could 'fix' this problem of eventual honesty for the visual system by demanding that the destination folder be an open folder (probably too restrictive, given the limited screen size) or by having the destination folder temporarily open up to reveal that the file is now located within it. This last suggestion is also a bit tricky, for we might still want to determine that the file no longer resides in the original folder, so we would have to be sure that the new folder did not obstruct the view of the old folder. In practice, this might be too difficult to guarantee in general.

4.2 It has been suggested in this chapter that consistency could be considered a major category of interactive principles, on the same level as learnability, flexibility and robustness. If this had been the case, which principles discussed in this chapter would appear in support of consistency?

answer

The discussion of consistency suggested that it can take many forms, because it is usually referred to in relation to some other feature of the interaction between user and system. Mentioned already in the text we have consistency related to the following principles:

• Familiarity - consistency with respect to prior real-world experience
• Generalizability - consistency with respect to experience with the same system or set of applications on the same platform

• Affordance - consistency with understood intrinsic properties of an object, so a soft button on the screen should allow us to always 'push' on it to select some action
• Predictability - consistency of system response with user's expectation, given the user has some information about past interaction history
• Substitutivity - consistent permission from system to allow use of equivalent values for input and output
• Commensurate effort - consistency of effort with respect to doing and undoing tasks
• Response time stability - consistency of system response for similar actions

Some other principles for consistency from the text and elsewhere:

• Consistency can be relative to the form of input/output expressions relative to the user's conceptual model of the system. An example in the text involves using keys whose relative positions are similar to commands for the systems (any set of four typewriter keys that form a diagonal to indicate up, down, left and right information for an input command).
• As discussed in the exercise on colour, consistency can be with respect to social or cultural conventions (e.g., using red to indicate stop or hot, green for go, blue for cool).

Worked exercises

• Discuss the ways in which a full-page word processor is or is not a direct manipulation interface for editing a document using Shneiderman's criteria. What features of a modern word processor break the metaphor of composition with pen (or typewriter) and paper? [page175]

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