Methodology I: Task Analysis - Foundations for Designing User-Centered Systems - page 308

describe how goals and subgoals are achieved, cover aspects such as the (possibly
conditional) sequencing of subgoals, the need for concurrency, and how cued
actions can direct performance.
11.3.2 Example Application of HTA
Table 11.2 shows a simple HTA for making an online purchase. It is not complete,
but illustrates the major steps, and would be useful when talking about how to
perform the task, and how to help users perform the task. It is complete enough
that it could be used to start a design. The plan at the bottom of the table is an
example, high-level plan. The plan—plans are usually numbered, with plan 0
being the plan associated with the top level goal—suggests that 1 must be com-
pleted before 2. There would also be plans for the subgoals, and sub-subgoals as
necessary. There might also be other plans for different users and uses.
Examining the HTA in Table 11.2 , we can start to identify some suggestions
for design. If you wanted to provide your users with the option of browsing the
listing before logging in, for example, you would have to change the plan to allow
task 2 to happen before task 1, as well as afterwards, so the revised plan would
look something like:
Plan 0: 1 or 2, then 2 as needed, then 1 if needed, then 3.
Figure 11.2 shows the HTA (this time drawn as a graph) for getting a file from a
web-based email system. A plan for downloading the first file would include all the
sub-tasks (1.1-1.4). A plan to download a second file could skip sub-tasks 1.1 and 1.2.
The analysis in Fig. 11.2 can also suggest changes to the design of the way
that the task is performed. Sub-task 1.2.2, for example, could be made more
general, allowing that users might move and click, and that CR might be
accepted as a way to move between fields. Further, it may be possible to skip
some sub-tasks in particular circumstances. If the browser remembers the user's
details then sub-task 1.2 could be skipped. Similarly, if the mail client downloads
the files automatically (like many mail clients), sub-task 1.4 can be skipped. The
details about skipping sub-tasks would normally be described in the associated
plans.
Performing an HTA can be a time-consuming process involving many
choices. Although it yields a useful representational description, the technique
does not specify how to go about gathering the task information that you wish to
represent.
There is no hard and fast rule about the lowest level at which you should
terminate the analysis. In general you should consider stopping at a natural level,
where further analysis would not contribute any more to your understanding of the
situation. Usually it is fairly obvious when to do this. More formally, you can
consider using the so-called stopping rule. For this you need to assess the prob-
ability, P, of inadequate performance and the cost, C, of inadequate performance if