A Practical Guide to the System Usability Scale (SUS)

Download a 150 page guide that provides background, benchmarks and best practices for using the System Usability Scale.

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This guide can also be purchased as part of the SUS Calculator Package for a discount.

Features & Benefits

The guide was assembled from 10 years of research and 25 years of data from over 5000 users. It contains the most comprehensive guide to using the most popular questionnaire for measuring perceptions of usability.

How to Interpret Your SUS Score
How to Compare your SUS Score to a Benchmark or other samples
Converting a SUS score into a Percentile Rank and Letter Grade (A+ through F)
Computing The Appropriate Sample Size
Discussion on Reliability & Validity
Acceptable Modifications to SUS
Common Mistakes and How to Avoid Them

Who Should Buy

This guide is both a "How to Guide" for those new to SUS and a comprehensive reference for experience researchers. It contains the most comprehensive data on SUS benchmarks, the latest research on reliability, validity and the only place you'll find information on statistical comparisons of SUS scores and how to compute appropriate sample sizes.

Highlights
The System Usability Scale
Background

SUS History & Development

Where did SUS come from?
How was SUS developed?

Scoring & Reporting SUS Scores

Scoring the SUS
Error Checks in the SUS Calculator
An Example Scored SUS Questionnaire
Should you report the mean or the median SUS Score?
What are possible SUS scores?
Handling missing values

Interpreting SUS Scores

Interpreting a SUS Score
SUS Grading on a Curve
Is a SUS score a percentage?
What is a good SUS score?
Can you report statistics on the individual items in the SUS?
Measuring changes in SUS over time

Properties and SUS Characteristics

Why are the items alternating between positive and negative wording?
Problems caused by alternating SUS items from positive to negative
Can SUS be used on websites?
Does SUS need to be administered during a usability test?
Does SUS provide an absolute measure of usability?

SUS Benchmark Data

SUS benchmarks by interface type
Summary Table of SUS Scores by Type of Interface
Definitions of benchmark sources
Source Table
Converting raw SUS score to percentile ranks

Modifying the SUS

Changing the wording of SUS items
Using an all-positively worded version of SUS
Adding items to SUS
Removing items from SUS
Changing the order of SUS Items
Changing the number of response options
Can I remove the neutral response?

SUS Reliability, Validity & Effectiveness

Does SUS have enough points of discrimination?
Are 10 items enough to measure usability?
How reliable is SUS?
How valid is SUS?
What other questionnaires are like SUS?
How SUS compares to other questionnaires
SUS & SUMI
SUS and WAMMI
Do SUS scores correlate with other usability metrics?
SUS and customer loyalty
Is SUS data normally distributed?
Detecting non-normality
The central limit theorem
Robustness
Statements about the population
Normality summary
SUS is free: Do you get what you pay for?
Does SUS measure usability?
What else does SUS measure?
Is SUS reliable in different languages?
What factors affect the SUS scores?
Experience
Age
Gender
Education
Accounting for levels of experience when comparing products with SUS
Is SUS the best questionnaire to measure usability?
Common mistakes when using the SUS

Statistical Comparisons of SUS Scores

Confidence intervals around a SUS score
Comparing a SUS score to a benchmark
Assumptions
How do I generate a percentile score relative to a benchmark?
Determining whether SUS scores are significantly different
Within-subjects comparison (paired t-test)
Confidence interval around the difference
Between-subjects comparison (2-sample t-test)
Why you don�t need to worry (too much) about violating assumptions

Sample Size

What is the minimum sample size needed?
Is a SUS score from a Small Sample Size Useless?
Standard deviation of the SUS
Calculating sample sizes for single SUS tests
Calculating sample sizes for comparing SUS scores
Between-subjects
Within-subjects
Calculating sample sizes for comparing a SUS score to a benchmark
One-sided confidence levels
Description of Data Sources
References
The System Usability Scale
All Positive Version of the System Usability Scale
About the Author