- The System Usability Scale, or SUS,
is a very popular survey instrument.
It was developed in the 80s by John Brooke at DEC.
Since then, it has been used on a wide range of systems,
from hardware to voice response technology
to consumer software.
The SUS includes 10 questions
answered with a five point Likert scale
from strongly disagree to strongly agree.
Note that the statements in this questionnaire alternate
the odd-numbered ones are positive,
and the even numbered ones are negative.
This is done for two reasons.
First, to prevent respondents from cheating
by selecting the same answer to all questions,
and second to counteract people's natural tendency
to agree with statements rather than disagree with that.
Because the SUS has many questions
that target the overall experience with the system
it is used at the end of a test.
After the user has spent time interacting with a site,
and doing usually more than one task.
The main use of SUS is in quantitative usability testing.
It enables us to compute a score for the site,
and because there's all that SUS data
collected from a variety of sites and applications over time
it also allows us to compare our site with others,
and understand where we stand on a global usability scale.
How is the SUS scored?
When you're scoring it, you're not just averaging
the scores from each question.
First, you'll be shifting the scale
to be from zero to four instead of one to five.
For odd questions which are positive,
you just subtract one from the response.
So response of one will become zero and five will be four.
Next, for the even questions which are negative,
disagreeing to them is good.
So you wanna give a high score to disagreement.
Basically, a strongly disagree response should be a four,
and a strongly agree one should be a zero.
Now, for each question, you've got a score
from zero to four, you have 10 questions,
so your range of scores will be from zero to 40,
but people usually like ranges from zero to a hundred.
These remind them of percentages.
So to convert a zero to 40 range into a zero to 100 one,
you simply multiply it by 2.5
because 2.5 times 40 equals 100.
That will give you a range from zero to a hundred
for the SUS score.
So let's assume that you've run a study, and at the end,
you've administered the SUS to all participants.
For each participant, you use this procedure
to calculate their SUS score.
Then you will take an average of all their scores.
For that average, you will calculate a confidence interval
in order to understand how well the score obtained
from your study predicts the SUS
over your entire target population.
Okay, say we've done all that work.
What is a good SUS score?
Because there's all that data about the SUS,
and because the SAS is supposed to be used
at the end of a test, after the participant has experienced
a website doing more than one task,
we can compare SUS scores across sites,
and determine what a good score is.
There are several ways to look at and interpret SUS scores.
First, you can use percentile ranges.
If you have kids, you may be familiar with percentiles
from visits to your child's doctor.
They will tell you that your child is, say,
in the 90th percentile for high.
What does that mean?
It means that 90% of the kids of the same age are shorter.
Same with a SUS, so if your score was 78,
you are in the 80 to 84th percentile,
meaning that between 80 and 84% of the size out there
have scores lower than yours.
A score of 68 falls at the 50th percentile.
A score of 52 falls at the 15th percentile,
so it's worse than 85% of the other scores in the database.
Another way to look at a SUS is based on letter grades
from A+ to F, with A+ being the best and F of fail.
So a score of 80 corresponds to an A-.
To summarize, the SUS is a well-tested survey instrument
meant to capture the usability of a site.
It is quite lengthy and is usually shown
after the user has had
a substantial interaction with a site.
For instance, has done several tasks
in a quantitative usability study.
It is a self-reported,
and thus highly subjective measure.
To get a full picture of your system's usability
you should combine it with performance-based metrics
such as completion rates and task times.