Topics

Usability

Learning Outcomes

Required Reading

Systems in Use

Usability Factors

Benefits and Failures

Summary

Bibliography

  • Customise

Usability Factors

Usability is concerned with making computer systems easy to use and easy to learn (Preece et al, 1994). So we move beyond the one-dimensional term 'user- friendliness' to a range of qualities known as usability factors.

The International Standards Office (ISO) definition of usability consists of three factors (efficiency, effectiveness, satisfaction). It is written for office workstations; hence its rather narrow focus. The standard for office work with visual display terminals is:

 

the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use.

(ISO 9241, Part 11)

Effectiveness and efficiency are productivity factors. Satisfaction is how the user feels about their use of the system.


Required Reading

Please now read Nielson, Chapter 2: What Is Usability. In this first reading, Nielsen considers usability in terms of usability factors and positions it in the broader context of system acceptability.


Self-assessment Question 3

Why is 'user-friendliness' an insufficient view of usability?




The Language of Usability

There are many interdependent factors that contribute to the usability of interactive computer systems. These factors have a different meaning to different HCI experts. By considering usability as the user's view of the quality of the system when used in a specific context, we can determine six key usability factors:

  • effectiveness: how well the user achieves the goals they set out to achieve using the system
  • efficiency: the resources, such as time and effort, consumed by users in order to achieve their goals
  • flexibility: how adaptable is the system to variations in users’ tasks
  • learnability: the time and effort required to reach a required level of performance
  • safety: how well the user, third parties, or data are protected from harm
  • attractiveness: the extent to which the system is aesthetically pleasing.

These factors provide a language for discussing the usability of interactive computer systems. Rather than saying that a system is not easy to use, or not very friendly, you can determine the relative importance of each of the factors for the particular system in use and then informally assess its usability in terms of the factors in a systematic fashion.

Now that the language of usability has been covered, we can recast our discussion of walk-up-and-use systems and information appliances in terms of the six factors.

  • Learnability is probably the most important factor for both systems, because they are used by the untrained general public everyday.

  • Although walk-up-and-use systems need to be effective, efficiency is more important because of the pressure users often find themselves under when operating them. The reverse is true for information appliances, whose users normally have time to operate them but need to trust the appliances never to err, because errors may have severe consequences.

  • Attractiveness is also an important factor for both types of system, because sales and usage are likely to depend on it. In walk-up-and-use systems flexibility is undesirable, but it is quite important for information appliances.

  • Finally, safety is a relatively important concern. Typically, this factor is applied to the user - whether they can operate the system without coming to any harm or causing any harm to third parties. For example, the levels of radioactive waves emitted by these devices have always caused worry amongst users and so it must be controlled through careful design. However, we take a novel view of safety, extending it to include harm to data also. In this case, any system that manipulates data, such as a word processor or online shopping site, must be reliable, so that data is secure and safe from misuse and damage. In this sense, safety is not an important factor for these types of system, because they contain very little user data.

Self-assessment Question 4

For each of the following five interactive systems, consider the context in which it is used then, for each, rank in order of importance the six usability factors. This can be done by filling in the table below.

  1. Car radio/tape player
  2. Information kiosk located in a railway station - it provides local information
  3. Supermarket point of sales till
  4. The control room for the CCTV security system for a large office building - the security employee monitors a bank of television screens and can control the orientation of each camera
  5. Share dealing desk containing four screens providing output from real time financial applications.
 

Effectiveness

Efficiency

Flexibility

Learnability

Safety

Attractiveness

Car Stereo

Information Kiosk

Point of Sales Till

CCTV Security System

Share Dealing Terminal