01 / UNDERSTAND
What is the System Usability Scale?
John Brooke’s System Usability Scale is a ten item questionnaire assessing perceived usability after system experience. A five point agreement scale produces one composite score from 0 to 100.
The score is not a percentage of completed tasks, liked features or satisfied requirements. It measures an overall subjective judgement and cannot identify specific interface changes.
Why use System Usability Scale (SUS)?
A consistent questionnaire provides a repeatable subjective measure alongside task observations. Comparable study conditions can support version comparisons or tracking over time. Preserve the population and experience behind the number.
When to use it
USE IT WHEN
Participants have had enough relevant experience to judge the defined system.
A team wants a consistent subjective measure alongside usability observations.
A planned comparison can maintain or account for participant and task differences.
When to choose another method
Use Usability Testing to find and understand specific problems behind an overall score.
Use Think Aloud Protocol to explore expectations during interaction.
Use Survey Research for broader questions that SUS was not designed to answer.
What you’ll need
Define system, version, participants and preceding experience. Obtain canonical items from Brooke’s original publication or a suitably validated translation. Keep the standard item wording and order. Preplan timing, missing response handling and analysis. A convenience sample does not represent every user. When participants rate alternatives in one session, earlier exposure can change later ratings. Plan order and analysis rather than treating both measurements as unrelated observations.
What a meaningful SUS result includes
Keep the elements that help answer your question and support a decision.
The standard ten items and response scale, with language version documented.
What each respondent used, attempted and experienced before responding.
Consistent timing and instructions without coaching the answers.
Correct direction adjustment for every item before forming the composite.
A defined reference with relevant differences in participants and tasks considered.
The score, sample size, uncertainty and accompanying behavioural evidence.
02 / PUT IT INTO PRACTICE
How to administer and interpret SUS
Keep evidence and assumptions distinct. Revisit your work when new information changes your understanding.
01
Define the purpose
Specify description, comparison or tracking. Name population and system boundary.
A kiosk differs from the entire library service.
02
Prepare the instrument
Use a traceable standard questionnaire. Pilot presentation and capture without rewriting items.
The worksheet is not a replacement questionnaire.
03
Standardise exposure
Provide relevant use and record tasks, help, failures and prior experience.
Keep exposure comparable across releases.
04
Collect responses
Administer after use and before debriefing, following Brooke. Avoid coaching.
Check all ten responses before ending collection.
05
Score each response
Odd items: response minus 1. Even items: 5 minus response. Sum ten contributions and multiply by 2.5.
The result is a score, not a percentage. Contributions range from 0 to 4.
06
Summarise uncertainty
Report sample size, distribution, summary and uncertainty using a method suited to the sample size and score distribution.
A small average difference does not establish improvement.
07
Connect to action
Interpret alongside observed problems and context.
Individual items are not standalone diagnostic measures.
Example: reviewing a library reservation kiosk
Fictional scenario: a library tests a reservation kiosk. Participants search, choose a branch and review a request. They complete canonical SUS immediately afterwards, before discussion.
For arithmetic only, imagine one complete response has adjusted contributions totalling 28. Multiplying by 2.5 gives 70. This invented score is neither a study average nor evidence of a grade.
The team records assistance and confusion about branch availability. It would investigate this despite a favourable overall impression. Later comparisons require comparable exposure and justified analysis, rather than a higher number alone.
03 / BELOW THE SURFACE
Below the surface: a stable instrument cannot stabilise the study
Keeping the questionnaire constant does not keep the surrounding experience constant.
EXPOSURE
What experience was the person rating?
Name the tasks, assistance and system boundary.
COMPARISON
What else changed between studies?
Inspect participant, task and administration differences.
ACTION
What evidence tells us what to improve?
Connect the composite with observed problems.
The second study might recruit experienced participants, use easier tasks or provide more help. A score change belongs to the whole study context until design and analysis support a narrower explanation.
Changing kiosk information while recruiting library staff would complicate attribution. Document both changes.
External benchmarks also carry context. Cite their source and inspect population, systems and scoring. This guide supplies no universal passing score. Make decisions using uncertainty, consequences and observed barriers rather than turning a broad questionnaire into a release gate.
System Usability Scale (SUS) companion worksheet
The original two page worksheet documents SUS preparation and result review: exposure, administration, scoring and comparison limits. Obtain the canonical questionnaire separately; these worksheets contain no replacement items or translation.
Practical tips
Independently verify scoring on a complete response.
Document the questionnaire’s language and source.
Report uncertainty alongside the average.
Common mistakes
Calling a score of 70 a 70 percent usability rating.
Rewriting or dropping items while comparing results as standard SUS.
Attributing a score difference to design when participants or tasks also changed.
What you’ll get
A traceable measure of perceived usability under stated conditions. Use the score to inform the discussion, then use observation and a credible comparison to decide what should change.