Methodology II: Cognitive Dimensions and the Gulfs - Foundations for Designing User-Centered Systems - page 330

in ever paler shades of the same color). Some spreadsheets now provide the
capability to show child cells, thus making the dependencies less hidden.
A much richer form of hidden dependency occurs in many modern operating
systems, where several files are both generated and used. Applications other than
those that created them may be dependent upon these files, such as a pictorial
figure in a spreadsheet file, a graphics file used in a report, or a preferences file
stored somewhere away from its application and labeled with the name of the
company or user rather than the application. These dependencies are not visible—
deleting files from system and user preferences directories therefore becomes a
hazardous task. The corollary is that people's file stores get filled up with old,
unused files and you cannot easily tell whether or not they are used. The normal
solution, albeit one that strictly ignores the problem rather than solves it, is to
never delete any files from these directories. The reason this works as a solution is
that storage space is so cheap!
12.2.2 Viscosity
A viscous system is one that is resistant to change—even small changes can
require substantial effort. A classic example would be a word-processed document
in which the numbers used in the figure captions, such as ''Fig. 12.1 ,'' have been
typed explicitly by hand (rather than using the word processor's built in numbering
facilities). If you decide to introduce another figure before this in the document,
then all the existing figure numbers need to be changed manually too. Whilst this
may not be too difficult for the figures themselves, ensuring that all the references
to the figures in the text are updated to match the updated numbering can require
significant effort. If you introduce several figures in this way, one at a time, you
may decide to skip doing the updating of the numbering every time, and in the end
may forget to do it at all. TeX and LaTeX avoid this problem by doing it auto-
matically, and Word also provides tools for doing this, although these tools are
somewhat viscous and error prone.
In some circumstances viscosity can be beneficial for users. If things are hard to
change this encourages reflective action and explicit learning. Learning in some
puzzles (noted in Chap. 6 ) is more explicit if the user has to work harder to make
moves in the puzzle. Deleting files is also made somewhat more viscous by many
operating system interfaces (''Are you sure you want to delete this file?''). In
contrast, if things are easy to change this encourages tinkering, which leads to
implicit learning. When it is very easy to make small changes this can often lead to
many small, unnecessary changes being made.
On the other hand, visualprogramming languages, one of the original analyses
that led to the CD approach, can have changes that are difficult to implement and
change. For example, it is most productive in most cases to vary fonts uniformly
and to have them vary by several points. It is not as productive to have them vary