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182 Chapter 6 The process of interaction design

et al., 1988; Nielsen, 1993). Usability engineering involves specifying quantifiable
measures of product performance, documenting them in a usability specification,
and assessing the product against them. One way in which this approach is used is to
make changes to subsequent versions of a system based on feedback from carefully
documented results of usability tests for the earlier version. We shall return to this
idea later when we discuss evaluation.

Consider the calendar system that you designed in Activity 6.1. Suggest some usability crite-
ria that you could use to determine the calendar's quality. You will find it helpful to think in
terms of the usability goals introduced in Chapter 1: effectiveness, efficiency, safety, utility,
learnability, and memorability. Be as specific as possible. Check your criteria by considering
exactly what you would measure and how you would measure its performance.
Having done that, try to do the same thing for the user experience goals introduced in
Chapter 1; these relate to whether a system is satisfying, enjoyable, motivating, rewarding,
and so on.

Comment Finding measurable characteristics for some of these is not easy. Here are some suggestions,
but you may have found others. Note that the criteria must be measurable and very specific.
Effectiveness: Identifying measurable criteria for this goal is particularly difficult since
it is a combination of the other goals. For example, does the system support you in
keeping appointments, taking notes, and so on. In other words, is the calendar used?
EBciency: Assuming that there is a search facility in the calendar, what is the response
time for finding a specific day or a specific appointment?
Safety: How often does data get lost or does the user press the wrong button? This may
be measured, for example, as the number of times this happens per hour of use.
Utility: How many functions offered by the calendar are used every day, how many
every week, how many every month? How many tasks are difficult to complete in a
reasonable time because functionality is missing or the calendar doesn't support the
right subtasks?
Learnability: How long does it take for a novice user to be able to do a series of set
tasks, e.g., make an entry into the calendar for the current date, delete an entry from
the current date, edit an entry in the following day?
Memorability: If the calendar isn't used for a week, how many functions can you re-
member how to perform? How long does it take you to remember how to perform
your most frequent task?
Finding measurable characteristics for the user experience criteria is even harder, though.
How do you measure satisfaction, fun, motivation or aesthetics? What is entertaining to one
person may be boring to another; these kinds of criteria are subjective, and so cannot be
measured objectively.

6.4 Lifecycle models: showing how the activities are related

Understanding what activities are involved in interaction design is the first step to
being able to do it, but it is also important to consider how the activities are related