SUS: The System Usability Scale

The System Usability Scale (SUS) has long been a favorite quantitative tool among usability researchers for its simplicity and accuracy. With just 10 questions, a 5-point “Strongly agree” to “Strongly disagree” response system, and a quick scoring algorithm, SUS yields an extremely reliable overall usability score for your website or product on a scale of 0 to 100.

SUS puts your website under the microscope

The questionnaire consists of the following 10 items:

  • 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.

Interpreting your System Usability Scale results

By various accounts, the mean SUS score hangs around 68-70.5. Some researchers have attempted to map adjectives like “excellent,” “poor,” or “worst imaginable” to individual scores for extra insight; by this system, an average score corresponds to the “good” range, though quite short of “excellent.”

With thousands of previously documented uses to compare to, your raw SUS score can be contextualized with a percentile ranking, allowing you to understand how your site performs relative to the wider web environment. Normalizing score distribution with percentiles therefore makes a 68 (or a 70.5) into a 50% – better than half of all other systems tested, and worse than the other half.

SUS System Usability Scale Scores Distribution

Though described by its inventor as a “quick and dirty” measure, studies have found SUS to be among the most accurate and reliable of all usability surveys, across sample sizes. It has today become one of the most successful metrics for quantifying system satisfaction, with thousands using it to gauge user-friendliness over a wide range of products online and off.

It is these qualities that make SUS the key to standardizing a diverse array of tester feedback, and aggregating responses into a meaningful but concise picture of your UX. If individual test videos are the trees, SUS shows you not only the forest, but the entire ecosystem into which your site fits; with a widely-trusted industry standard to rely on, you can take a step back from your own company and see how you chalk up in the broader world that surrounds you.

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