Human-Computer Interaction and Presence in Virtual Reality

Chapter 2
ISO 9241 Nielsen
Efficiency
Efficiency
Learnability
Memorability
Effectiveness Errors/Safety
Satisfaction Satisfaction
Table 2.1: Usability as in (ISO 9241) and (Nielsen, 1993), adapted from (van Welie, 2001)
The usability aspects defined by Nielsen provide a practical operationalisation that we will
use in this dissertation. In literature, several methods have been proposed for determining the
usability of a system, which will be reviewed in the next paragraph.
2.3.2 Determining usability
A wide range of different methods for performing usability evaluations have been proposed,
each with its own body of advocates (Holyer, 1993). Unfortunately, even though attempts
have been made to compare these methods, most of these are hampered by inadequacies in
the design of these studies (Gray & Salzman, 1998).
Usability evaluation can take place by investigating conformity of the design to heuristics
and guidelines. Another possibility is to test one or several prototypes with real users.
Usually, several evaluation methods are combined. For instance, Hix et al, when developing
a VE for battlefield visualization, used heuristic evaluation followed by user evaluation of a
single prototype, and summative evaluation, where users are required to compare two
designs (Hix et al., 1999). The main reason for first performing heuristic evaluation is that
this type of evaluation is relatively cheap; no prototype had to be constructed.
Heuristic evaluation
Heuristic or expert evaluations are performed by usability experts who check the design for
possible usability problems. Guidelines often form the basis of such an evaluation, and these
will be discussed in more detail in the next paragraph.
Another method for heuristic evaluation is the use of a cognitive walkthrough. Here, the
usability experts will go through the design following a scenario of a user working with the
system, often depending on the expertise of the evaluator to identify possible usability
problems. This process can even be automated using a Programmable User Model (Amant,
1999) that evaluates whether for each task there is a chain of user interface commands that
will lead to the successful completion of that task.
The proposed task situation can be modeled in more detail using a TA method. This model
can then be used to make predictions about the usability and the performance of the design.
Several methods apply techniques reminiscent of the time studies used in the Scientific
Management method (Taylor, 1911), extending these to cognitive labor as well as physical
labor. For instance the GOMS method includes a Keystroke-Level model (Card et al., 1983)
that can be used to predict the time required to complete a certain task.
Guidelines and principles
Guidelines and principles can help the evaluator in applying existing usability knowledge to
the design. Principles are high-level usability requirements and are too general to be applied
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