SUS (System Usability Scale) is a commonly used questionnaire-based method for assessing the perceived usability of a system or product. It provides insights into the user's overall satisfaction and perception of usability.

Steps for Creating a Successful SUS Test

To conduct a SUS test, the Nielsen Group recommends the following steps:

  1. Prepare the questionnaire: The SUS questionnaire consists of 10 statements related to the usability of the system. Participants rate each statement on a 5-point Likert scale, ranging from “Strongly Disagree” to “Strongly Agree.”
  2. Administer the questionnaire: Participants rate their agreement with each statement based on their experience using the system.
  3. Calculate the scores: For odd-numbered items, subtract 1 from the participant’s score. For even-numbered items, subtract the participant’s score from 5. Sum up the adjusted scores for all 10 items, resulting in a score between 0 and 40.
  4. Interpret the results: The SUS score is used to compare different systems or track usability improvements over time. Higher scores indicate better usability, with a score above 68 generally considered above average.

It’s important to note that SUS is just one of many methods used to evaluate the usability of a system. It should be complemented with other UX research techniques for a comprehensive understanding of the user experience.

Questions in a SUS Test

The following are the questions asked in a SUS test:

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

Results and Interpretation

The SUS scores are interpreted as follows:

  • 80.3 or above: A (Excellent)
  • 68 to 80.3: B (Good)
  • 68: C (Okay)
  • 51 to 68: D (Poor)
  • Below 51: F (Awful)

Cronbach’s Alpha Formula

The formula to calculate Cronbach’s alpha reliability coefficient for SUS is as follows:

  1. Calculate the correlation between each pair of questionnaire items.
  2. Sum all the obtained correlations.
  3. Divide the sum of correlations by the total number of questionnaire items.
  4. Subtract the result obtained from 1.
  5. Divide the result obtained by the number of questionnaire items minus 1.

The resulting value of Cronbach’s alpha provides a measure of the internal consistency of the questionnaire. Higher values indicate greater reliability and consistency in participants’ responses.

It is important to note that Cronbach’s alpha is a statistical technique used to evaluate the reliability of items in a questionnaire. It measures the internal consistency of the items, rather than the validity of the questionnaire itself.

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