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ConsistencyInDesign
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1994-10-22
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CONSISTENCY IN DESIGN
Bruce Tognazzini
Product Engineering Apple Computer, Inc.
Achieving Consistency in a stable system is a difficult enough task.
Trying to do so in the rapidly evolving world of computers can be
positively Herculean.
An important aspect of consistency was defined by Smith et al. [1983]
in this way: "Consistency asserts that mechanisms should be used in
the same way wherever they occur." I would add, by inference, "and
whenever they occur": first release or fifth release.
Well, that makes it all rather simple, doesn't it? Just don't change
anything, and you will be able to maintain perfect consistency!
Unfortunately, the computer industry is still in its infancy. Before
we can polish and fine-tune the current generation of a given system
or application, we are often driven by market pressure to release the
next generation, with new metaphors, objects, and behaviors. With so
much movement, how can we hope to maintain complete consistency? We
can't. Instead, we must pick and choose which aspects of consistency
are most important to maintain. Many principles can help guide those
choices. Some are obvious; for instance: Follow published guidelines
whenever possible. Don't change something unless it really needs
changing. Add new skills to the user's skill set rather than expecting
the user to modify existing skills.
A few principles appear to be less obvious. It is these I have chosen
to highlight.
Two Principles for Change
The following two principles, taken together, offer the designer
tremendous latitude in the evolution of a product without seriously
disrupting those areas of consistency most important to the user:
1. Consistent interpretation of user behavior by the system is more
important than consistent system objects or behaviors.
The computer system sends information to the user via text, graphics,
sound, etc. The user sends information to the computer via the
keyboard, mouse, etc. In both cases, the receiver of the information
applies specific interpretations to the sender's message:"He pressed
the second key on the third rank of the keyboard. I'll place an 'A'
character in the text buffer and on the display...." In the real
world, the content of the communication between a given sender and
receiver changes often. The sender's method of transmission will also
change, but seldom. The receiver's interpretations will rarely, if
ever, change: the receiver will not unilaterally decide to interpret
time as "time stated minus one-half hour," then yell at his or her
spouse for arriving half an hour late. If the system as sender acts in
some new way, people may at first be startled, but they will soon
learn how to interpret that new behavior. If a system object has
changed appearance, people do not go into a blind panic—they learn the
meaning of the new appearance. But if the system suddenly interprets
the user's pressing Command-R, which used to mean "readjust the
margin," to mean "remove everything from my hard disk," the user is
going to be more than a little upset.
2. If you must make a change, make it a large and obvious one.
Designers often fear making changes that are highly visible. Yet these
are just the sort of changes that cause no confusion: Faced with an
entirely new appearance for a program, people will just sigh and begin
learning what's new. But inject the word "not" into the question, "Do
you want to save this document before closing?" and it could take the
user several lost documents and a few trips to the dealer to trace
down the real culprit: the designer. What starts out as a conscious
skill quickly becomes automatic. Questions such as, "Do you want to
save . . . ?" are soon no longer read. The "OK" button is clicked or
Return key pressed with no conscious thought. Even after the user
realizes that the computer has presented a subtly new question, the
task of adapting to the change has only begun. Now he or she must
somehow block the automatic response long enough to unlearn the old
habit and acquire the new.
A Combination of Ingredients
Taken together, these two principles offer the designer a great deal
of freedom: You can change the entire look and feel of an application
as long as you honor the user's previously learned interpretations and
subconscious behaviors.
Put in oval windows. Change from a dialog box offering a list of
specifications to a directly manipulable object. Go from 2D to 3D.
Just don't make the left-arrow key make the pointer go to the right.
Don't shuffle all the tools around in a palette so that the user
depending on spatial memory keeps selecting the wrong one.
Consider your car. Let's say you got it back after its 3O, OOO mile
service and found they had added anti-lock brakes and hidden,
retractable wings, all for under $1OO. You would probably be quite
happy. But now let's say that they switched your gas and brake pedals
and didn't charge you anything at all! Think you'd be thrilled? And
yet, it is so easy for designers to change the "pedals" around on the
keyboard that it's done all the time. Consistency also derives from
what users expect to happen as a result of their actions. Coined by
W.A.S. Buxton, the term consistency with expectations implies a
principle for those occasions when consistency seems inconsistent, or
vice versa. Before any product ships, take time to do user testing
with the target population. Find out what users expect, what would
stimulate them to change their expectations, and what would make such
a change worth the trouble.