The System Usability Scale (SUS) is a questionnaire which can be used to help evaluate the usability of a project. The questionnaire is used as a quantitative analysis method within the framework of usability tests. The SUS consists of only 10 questions, which are answered using a scale from 1 to 5 (Lickert scale). The range goes from "I agree completely" to "I do not agree at all".
The SUS was developed by John Brooke in the 1980s. At that time he worked for the US company Digital Equipment Corporation in Great Britain. He presented the questionnaire for the first time in 1986 as part of usability tests for electronic office systems.
To this day, the System Usability Scale is still used to evaluate the usability of software or websites, as the tool can be implemented quickly and easily. The inventor of the SUS also called it "a quick and dirty usability scale" according to the title of one of his essays.
To develop the questionnaire, Brooke had selected 10 concise statements from 50 different statements that he had previously made available to a 20-strong group for software testing between two systems. Due to the great diversity of tested software systems, Brooks was able to select those 10 statements that most clearly emphasized the differences between the systems.
The System Usability Scale is used in a variety of usability tests. The following scenarios are common.
Usually, the SUS is applied after a usability test has been carried out. On average, participants only need a few minutes to answer the questions.
Each questionnaire contains 10 questions, so-called "items" for which the test person has a rating scale of 5 fields to mark. Individual statements cannot be deleted, as the subsequent evaluation will not work otherwise.
The statements of the SUS are as follows:
# one: I think that I would like to use this system frequently
# two: I found the system unnecessarily complex
# three: I thought the system was easy to use
# four: I think that I would need the support of a technical person to be able to use this system
# five: I found the various functions in this system were well integrated
# six: I thought there was too much inconsistency in this system
# seven: I would imagine that most people would learn to use this system very quickly
# eight: I found the system very cumbersome to use
# nine: I felt very confident using the system
The result of a survey using the System Usability Scale is the so-called SUS score. This is a number between 0 and 100, with 0 representing the worst possible result. For the evaluation, only one questionnaire is considered at a time. Individual items are not sufficiently meaningful to arrive at a quantitative evaluation.
Five responses are available to the user for the evaluation of the individual items. For the evaluation, the most negative answer is assigned a 0 and the most positive a 4. The negative formulated answers are coded with 0 to -4.
Once the values for each individual item have been determined, they are added together. In this way, the tester receives a number between 0 and 40, which in turn is multiplied by 2.5, resulting in the SUS score. This figure is usually expressed as a percentage.
If a system has reached 100 percent, it is perfect. Values of up to 70 per cent are considered good usability, and less than 50 per cent are said to be seriously flawed in terms of usability.
The System Usability Scale is a quick method to determine the usability of a product or software. The SUS can thus provide a good basis for further planning and large-scale usability tests. However, the test method is not sufficient to obtain precise information on the respective defects of the product.