Methodology II: Cognitive Dimensions and the Gulfs - Foundations for Designing User-Centered Systems - page 339

reduce the size of the Gulf of Execution and the Gulf of Evaluation. Table 12.3
provides some general guidance on how you can try to narrow the Gulfs during
design.
Visibility is not an adequate guideline on its own, however, and making too
much information visible can be a bad thing. Norman uses the example of a
washer/drier that had multiple controls and several symbols displayed on the front
of it. There was no easy way to discern what each control did and what all the
symbols meant without looking at the instructions or undergoing extensive
training. (This example assumes that the designer was not trying to drive the user
to the manual to learn these symbols, a type of controlled viscosity.)
There is often a design trade-off between presenting too much information and
overloading controls to allow them to perform several different actions using
modes. In moded systems, the way that you interpret the displays and feedback
depends on which mode the system is currently in. This can be problematic,
particularly if it is not immediately obvious which mode the system is in.
The visibility guideline, like most guidelines, needs to be adapted to the user's
particular situation. You should think of it in terms of:
''Visibility of the appropriate information for carrying out relevant tasks,'' or
''So viel wie nötig, so wenig wie möglich.'' ? ''as much as necessary, as little as
possible,'' is a neat way of phrasing it, taken from a German aphorism.
What counts as appropriate information will vary across tasks, and sometimes
across users, and even across contexts of use (e.g., same driver, same task, dif-
ferent road conditions).
When you think about visibility you should make sure that you give appropriate
consideration to feedback, consistency, and the user's mental model of the system.
These factors can all be used to help reduce the size of the Gulfs. Feedback is
obviously important in helping to reduce the Gulf of Evaluation, because it shows
the effect of performing a particular action.
Users often rely on consistency within and across systems and applications.
They expect interface layouts to be consistent, for example, with particular con-
trols always being located in the same place and always behaving in the same way.
This can be particularly important when it comes to new systems, because users
will often try to apply their knowledge of other interfaces to the new interface.
Consistency is important because it will help the users to help themselves.
We know that users rely on mental models to help them perform tasks. These
mental models develop over time, so your design should facilitate the development
of appropriate mental models, and support the use of those mental models by
making the appropriate information visible to users at the right time and in the
right place.
In many cases you want it to be easy to use the system you are designing, so
that you can get to the point where people can use it with little or no conscious
control. In other words, they will be operating at a point close to Rasmussen's
( 1983 ) skill-based level of behavior.In critical systems, however, you often want
people to pay close attention to the actions that they are taking, and for behavior to