Summary: Putting It All Together - Foundations for Designing User-Centered Systems - page 377

Number of Problems Attempted

Time to Push a Button

Fig. 14.1 A normal curve (left) [Strictly speaking, number of problems cannot go below 0 either,
but for most practical examples, this distribution will be well represented by a normal curve when
0 is more than three standard deviations from the mean (the peak).] and a gamma curve (right)
showing typical distributions for tasks attempted and response times for a task. These curves
illustrate that users will have a distribution of behavior, and not always perform a task or have
knowledge in a single way

experts is that expert users can often (but not always) detect and correct errors
before any adverse consequences arise. When you design systems, you need to
remember that all users will make errors. Some of these errors will be preventable,
but others may not. You will therefore need to decide which errors you will
prevent (using risk assessment, for example). For the other errors, you will need to
include support to help the user spot errors and, in particular, to support the ability
to correct errors, ideally before any adverse consequences arise.
Where the users have physical limitations, however, or there are contextual
limitations (low lightinglevels, for example), these limitations will constrain what
the users can and cannot do.

14.2.3 Cognition

Users' cognition is limited, at both the tactical level (e.g., limited working
memoryand some difficulties in storing and retrieving items from memory) and at
the strategic level (e.g., how to decide which are the important and long term
issues), as noted in Chaps. 5 - 7 . In some cases, the effects of these limitations can
be ameliorated through learning, or through social processes, particularly where
skills are pooled to perform particular tasks.
Attracting, managing, and maintaining the user's attention are all important.
Users have limited attentional resources. This means that in busy contexts, where
lots of things are happening or several tasks have to be performed simultaneously,
they are unlikely to be able to attend to all of them. In other words, they will find it
difficult to consciously control how they perform all the tasks. With practice, users
can learn to perform some tasks with little or no conscious control (i.e., with little