What is Usability?

11.2 What is Usability?

Usability represents one of the most important acceptance criteria for interactive applications in general and – as we will see soon – for Web applications in particular. Unfortunately, this situation is analogous to human health; people notice it only when it is absent. In software, this unavoidably leads to frustration for users, to their inner revolt against the product, and most likely to their refusal to use it. This is particularly true if there are alternatives, which is usually the case with Web applications: a hotel reservation system is normally only a few clicks away from a competitive product.

Before we look at how usability can be ensured in Web applications, we need to understand what usability actually means. Usability can be defined as “the extent to which a product can

be used by specified users within a specified usage context to achieve specified goals effectively, efficiently, and satisfactorily”. This rather brittle definition in the international ISO/IEC standard 9241-11 actually describes the most important aspects to be considered when designing usable software: Who are the product’s users? What are the product’s goals? In what context is the product used? Can the goals be achieved effectively (i.e., completely and exactly), efficiently (at

a justifiable cost relative to the achievement), and satisfactorily? “Satisfactorily” means that the use is free from impairments and results in a positive user attitude towards using the product. The latter aspect is greatly influenced by a Web application’s aesthetic appeal (see section 1.3.1).

The systemic character of the above definition points to the fact that it is by no means sufficient to consider general rules of thumb in the design of usable applications. What we need to do is to consider goals, users, and the use context in each individual case, and to control the development

2 The error rate in online purchasing is about 30% (The Boston Consulting Group 2000). Only 1% of the visitors to an e-commerce Web site managed to actually buy something in 1999 (Nielsen 1999c). In 2005 the average rates have risen to about 3% while top rates are about 15% according to ( http://www.conversionchronicles.com/ ), citing e.g. Nielsen/NetRatings.

11.2 What is Usability? 221 process by appropriate feedback mechanisms. Thus we can reach the multi-dimensional target

without overshooting. In other words, being able to state single product characteristics beforehand would be nice since it would guarantee usability virtually automatically, but it is impossible due to the complexity of this undertaking. In contrast, the desire to achieve usability in the final product should be understood as an ongoing series of actions that should be consistently integrated in the application development process. This approach leads to what is known as usability engineering, which is the main topic of this chapter.

There are several approaches to specify the three criteria – effectiveness, efficiency, and satisfaction – in more detail. While Nielsen (1994) and Shneiderman and Plaisant (2005) iden- tify metrics to support a concrete evaluation of each of the ISO criteria (Table 11-1), van Welie et al. (1999) describe additional mechanisms that can positively influence these metrics.

Table 11-1 Usability models according to Nielsen and Shneiderman

ISO/IEC 9241-11

(Nielsen 1994)

(Shneiderman and Plaisant 2005)

Efficiency

Efficiency

Speed of performance

Learnability

Time to learn

Memorability

Retention over time

Effectiveness

Rate of errors by users Satisfaction

Errors/safety

Satisfaction

Subjective satisfaction

Errors/Safety

Satisfaction

Usage Indicators

Performance Speed

Feedback Undo

Means Warnings

Task Conformance

Adaptability

Knowledge

User Model

Design Knowledge

Task Model

has an impact on is a source for improving

Figure 11-1 Layered usability model according to van Welie et al.

222 Usability of Web Applications These mechanisms, in turn, are rooted in the knowledge of application-specific user and

task models. Figure 11-1 illustrates some of these conceptual interrelationships rather vividly. However, it should not lead to an oversimplified view.

The methods available for usability evaluation can basically be categorized into two classes. User-based methods involve end-users in the evaluation and optimization of a product’s usability during design, prototyping and implementation. In contrast, expert methods are based on the experience of experts and generally accepted “theoretical” models. A higher validity of the evaluation can generally be expected from user-based methods, provided they are based on a methodological and clean approach. However, they are much more time-intensive and costly. Most cases combine methods from both classes.