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CHAPTER 7
design rules
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EXERCISE 7.1
7.1 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.
EXERCISE 7.2
7.2 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
In addition, we could interpret some
other principles as contributors to consistency:
- 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).
EXERCISE 7.3
Find as much information as you can on
ISO standards that relate to usability. (Hint: many
standards are discussed in terms of ergonomics). How
many different standards and draft standards can you
find?
answer
open-ended research
EXERCISE 7.4
7.4 Can you think of any instances in
which the 'noun-verb' guideline for operations, as
suggested in the Apple human interface guidelines
for the Desktop Interface, would be violated? Suggest
other abstract guidelines or principles besides consistency
which support your example. (Hint: Think about moving
files around on the Desktop.)
answer
The noun-verb guideline suggests that
we can view all operations that the user will perform
as being composed of an action (the verb) acting with
one argument (the noun). In the case of moving a file
(or copying, for that matter), the action (move or
copy) requires more than one argument. The way the
move operation is performed requires the user first
to select the icon for the file to be moved and then
to indicate the move operation implicitly by dragging
the selected icon to the destination folder. The nouns
in this dialogue are the file to be moved and the
destination folder. The verb is the move operation.
The natural way to express this is in the order noun-verb-noun.
Strictly speaking, in order to stick with the noun-verb
guideline, we would have to indicate both the target
file and the destination folder before indicating
the move operation. That would be consistent, relative
to input expression, with most other commands on the
desktop. However, some principles of direct manipulation
and familiarity to the user are more important. Moving
files by dragging them on the desktop is very similar
to the way we can pick up any object in the physical
world and move it to its new location. And the dragging
operation is incremental and easily recoverable; moving
to one place can be undone within the same operation
since the dragging can continue until the file is
released.
The file-moving example is a slightly
contrived one, because some could argue that there
is no violation of the noun-verb guideline (hence,
moving is still consistent with respect to input expression)
because the verb is 'move to destination folder'.
Perhaps a better example is a command to search a
file system for files matching some specification.
Here, the action is to do the qualified search and
the argument or noun is the set of folders or volumes
of the system that you want searched. Typically, this
kind of operation is defined by some dialog box that
allows the user to indicate in any order the specifics
of the operation (the search parameters) and the folders
or volumes to search. Once this unordered dialog is
complete, the user then indicates that it is OK for
the system to perform the operation. This kind of
form-filling dialog subscribes to neither the noun-verb
or verb-noun guideline; the order is more flexible
for the user than consistent.
EXERCISE 7.5
Can you think of any instances in which
the user control guideline suggested by Apple is not
followed? (Hint: Think about the use of dialog boxes.)
answer
The user control guideline states that,
'The user, not the computer, initiates and controls
all actions.' In the case of dialog boxes, this guideline
is clearly contradicted. A dialog box can be used
to indicate when an error occurs in the system. Once
this error has been detected and presented to the
user in the dialog box, the only action that the system
allows the user is to acknowledge the error and dismiss
the dialog box. The system preempts the user dialog,
with good reason. The preemptive nature of the dialog
box is to ensure that the user actually notices that
there was an error. Presumably, the only errors that
will be produced in such an intrusive manner are ones
which the user must know about before proceeding,
so the preemption is warranted. But sometimes dialog
boxes are not used to indicate errors and they still
prevent the user from performing some actions that
they might otherwise wish to perform. The dialog box
might be asking the user to fill in some information
to specify parameters for a command. If the user does
not know what to provide, then they are stuck. A lot
of the time, the user can find out the information
by browsing through some other part of the system,
but in order to do that they must exit the dialog
box (and forfeit any of the settings that they might
have already entered), find out the missing information
and begin again. This kind of preemption is not desirable.
It is probably this kind of preemption the user control
guideline is intended to prevent, but it doesn't always
get applied.
EXERCISE 7.6
Find a book on guidelines. List the guidelines
that are provided and classify them in terms of the
activity in the software life cycle to which they
would most likely apply.
answer
We use as a source of guidelines Mayhew's
book Principles and Guidelines in Software and
User Interface Design [230]. In general, all guidelines
offer constraints on the design activity and so should
be known during the requirements phase. In the following
list, we will concentrate on what other stages (architectural
design, detailed design, coding and unit testing,
integration and testing) will be most affected by
the guidelines. The numbers in parentheses indicate
the page reference for the given guideline.
Architectural design
- Present functionality through a familiar
metaphor. (97)
- Provide similar execution style of
analogous operations in different applications.
(97)
- Organize the functionality of a system
to support common user tasks. (442)
- Make invisible parts and processes
visible to the user. (95)
Detailed design
- Consistent dialog style for different
functions. (97)
- Match menu structure to task structure.
(144)
- Create logical, distinctive and mutually
exclusive semantic categories with clear meanings.
(150)
- Design and organize a fill-in form
to support the task. (184)
- Consider voice synthesis as an output
device when the user's eyes are busy, when mobility
is required, or when the user has no access to a
workstation or screen. (427)
Coding and unit testing
- On full-screen text menus, present
menu choice lists vertically. (148)
- In a fill-in form, use white space
to create a balance and symmetry and lead the eye
in the appropriate direction. (186)
- Avoid frequent use of shift or control
keys. (256)
- Place high-use function keys within
easy reach of the home row on the keyboard. (281)
Integration and testing
- Allow full command names and emphasize
them in training, even if abbreviations are allowed.
(261)
EXERCISE 7.7
(a) Distinguish between principles, guidelines
and standards, using examples of each to illustrate.
(b) Why is context important in selecting
and applying guidelines and principles for interface
design? Illustrate your answer with examples.
answer available for tutors only
EXERCISE 7.8
(a) Why are there few effective HCI standards?
(b) How do "golden rules" and
heuristics help interface designers take account of
cognitive psychology? Illustrate your answer with
examples.
answer available for tutors only
EXERCISE 7.9
Using the web design pattern language
in The Design of Sites [356] produce a design
for an e-commerce site for a small retail business.
How well does the language support the design process?
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