SYSTEMS USABILITY SCALE

“Usability is not only about ease of use but also about bringing something meaningful, having an objective in common with the user.”

– Marcus Österberg, Web Strategy for Everyone

STANDARD VERSION OF THE SYSTEMS USABILITY SCALE

As per ISO 9241 Part, 11 – Usability of a system can be measured only by taking into account the context of use –

• Who is using the system? 
• What are they using it for
• The environment in which they are using it in

It also defines Usability as the extent to which a system, product, or service can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction in a specified context of use.

Thus, it is clear that a scale measuring the usability of a system, product, or service should measure 3 factors:

• Effectiveness (Can users achieve their objectives?)
• Efficiency (How easily can users achieve these objectives?)
• Satisfaction (Were users satisfied to use the system while achieving those objectives?)

However, the measures of effectiveness and efficiency are subjective. An example that Wikipedia mentions are:

Effectiveness in using a system for controlling a continuous industrial process would generally be measured in very different terms to, say, effectiveness in using a text editor.

Due to this subjectiveness, it is difficult to say that the former is better than the latter.

John Brooke’s Systems Usability Scale, developed in 1986, offers a high-level subjective view of usability thus allowing comparisons between two distinct systems.

ADMINISTERING THE SYSTEMS USABILITY SCALE (SUS)

The Systems Usability Scale is a Likert Scale that has 10 questions, requiring participants to rate each question from 1 to 5.  5 meaning that they strongly agree with the question, and 1 meaning that they strongly disagree.

The 10 questions are:

• I think that I would like to use this system frequently.
• I found the system unnecessarily complex.
• I thought the system was easy to use.
• I think that I would need the support of a technical person to be able to use this system.
• I found the various functions in this system were well integrated.
• I thought there was too much inconsistency in this system.
• I would imagine that most people would learn to use this system very quickly.
• I found the system very cumbersome to use.
• I felt very confident using the system.
• I needed to learn a lot of things before I could get going with this system.

SCORING ON SUS

After the participants have ranked each of the 10 questions, it’s time to determine the score of the system, product, or service on the scale.

• For each of the odd-numbered questions subtract one from the score allocated by the participant.
• For each of the even-numbered questions subtract five from the allotted score.
• When a participant fails to assign a score to a question, assign the score as 3.
• Add each of the new values arrived after the above subtraction/addition, and multiply the entire score by 2.5.

The result will be a score out of 100. It is important to remember that this score is not a percentage, but a clear way of seeing the score.

MEANING OF THE SCORE

A score of 68 out of 100 is the average score on the Systems Usability Scale. Any score below 68 is below average and it means that your system needs more work.

ONE-DIMENSIONAL

The SUS yields a single score on a scale of 100, thus being one dimensional. This single score can be beneficial to compare usability amongst completely dissimilar systems. 

Lewis and Sauroresearched to challenge the unidimensionality of the SUS. They concluded that questions 4 and 10 were testing a factor other than Usability as opposed to other questions. 

Question 4: I think that I would need the support of a technical person to be able to use this system.

Question 10: I needed to learn a lot of things before I could get going with this system.

The above two questions measure the Learnability of a System rather than the Usability of the system. 

Lewis and Sauro suggest that the SUS be used with the existing 10 questions, rather than dropping questions 4 and 10. This will result in a cleaner way to get an estimate of perceived Usability (with questions 1 to 3 and 5 to 9) and an advantage of getting an estimate of perceived Learnability (with questions 4 and 10)

John Brooke described the Systems Usability Scale as a quick and dirty way to check the usability of a system, product, or service.  Based on the above article it’s sufficient to say that it’s quick but far from dirty.

This article is a part of a series of articles. You can also check out our latest blog posts.