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102 Chapter 3 Understanding users

A main emphasis has been on transforming theoretical knowledge into tools
that can be used by designers. For example, Card et al's (1983) psychological model
of the human processor, mentioned earlier, was simplified into another model
called GOMS (an acronym standing for goals, operators, methods, and selection
rules). The four components of the GOMS model describe how a user performs a
computer-based task in terms of goals (e.g., save a file) and the selection of meth-
ods and operations from memory that are needed to achieve them. This model has
also been transformed into the keystroke level method that essentially provides a
formula for determining the amount of time each of the methods and operations
takes. One of the main attractions of the GOMS approach is that it allows quantita-
tive predictions to be made (see Chapter 14 for more on this).
Another approach has been to produce various kinds of design principles, such
as the ones we discussed in Chapter 1. More specific ones have also been proposed
for designing multimedia and virtual reality applications (Rogers and Scaife, 1998).
Thomas Green (1990) has also proposed a framework of cognitive dimensions. His
overarching goal is to develop a set of high-level concepts that are both valuable and
easy to use for evaluating the designs of informational artifacts, such as software ap-
plications. An example dimension from the framework is "viscosity," which simply
refers to resistance to local change. The analogy of stirring a spoon in syrup (high
viscosity) versus milk (low viscosity) quickly gives the idea. Having understood the
concept in a familiar context, Green then shows how the dimension can be further
explored to describe the various aspects of interacting with the information structure
of a software application. In a nutshell, the concept is used to examine "how much
extra work you have to do if you change your mind." Different kinds of viscosity are
described, such as knock-on viscosity, where performing one goal-related action
makes necessary the performance of a whole train of extraneous actions. The reason
for this is constraint density: the new structure that results from performing the first
action violates some constraint that must be rectified by the second action, which in
turn leads to a different violation, and so on. An example is editing a document using
a word processor without widow control. The action of inserting a sentence at the
beginning of the document means that the user must then go through the rest of the
document to check that all the headers and bodies of text still lie on the same page.