Measuring Usability
Quantitative Usability, Statistics & Six Sigma by Jeff Sauro

SUS Guide & Calculator Package

Download a calculator and a comprehensive 150 page guide. The calculator provides easy ways of scoring your SUS, comparing it to 500 other studies, statistical comparisons and sample sizes calculations. The guide explains everything you need to know about SUS and provides background information, benchmarks and best-practices.
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If you will be making copies or distributing the product, please purchase the site license
Educational and Student Discounts are available. The guide can be purchased (without the calculator) from Amazon or downloaded electronically.

Demonstration of the SUS Calculator



Features & Benefits

  • Score The SUS Properly: 10% of SUS questionnaires contain coding errors. The calculator includes several checks to prevent miscoding.

  • Converts SUS scores to Percentile Ranks: Compare your SUS score to data from 500 studies by application type (e.g. websites, consumer software, hardware, IVRs). For example, a percentile rank of 75% tells you your application has higher perceived usability than 75% of all applications in the database.

  • Converts SUS scores to Grades (A+ to F): Using my large database of studies, I converted the percentile ranks to letter grades. A grade of a "C" equals the 50th percentile (meaning average) and a grade of an "A" is any SUS score above an 80.3.

  • Compares SUS Scores Statistically: Compare Two SUS scores, a single SUS score to a benchmark and generate confidence intervals. There are calculations for between and within-subjects studies.

  • Handles Missing Values: Uses a substitute method for estimating missing values which can be essential when sample sizes are small.

  • Computes Sample Sizes: Instead of relying on general rules-of-thumb, the calculator computes the best sample size for all study-types.

  • Checks for Inconsistent Respondents: The calculator flags respondents who are agreeing to too many favorable and unfavorable responses. This suggests users weren't paying attention or there was a coding error.

  • Checks internal reliability: Cronbach's alpha is calculated from your raw responses. The calculator will tell you if the reliability of your responses is acceptable or poor.


Who Should Buy

If you use SUS, then you can benefit from this package. The 150 page guide is both a "how-to" guide for those new to SUS and a reference for experienced researchers. The calculator reduces errors in coding and provides meaning to your SUS score by converting raw scores into percentile ranks and letter-grades. All the necessary statistical calculations from comparisons to sample size estimation are included.

Guide Table of Contents

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 donít 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

Select Sample Screen Shots

Scoring, Reliability, Inconsistency Check & Missing Value Substitution

Scoring, Reliability, Inconsistency Check & Missing Value Substitution

Dashboard for Changes in SUS over time

Dashboard for Changes in SUS over time

Converts SUS scores to Percentile Ranks & Grades

Converts SUS scores to Percentile Ranks & Grades

Comparing Two SUS Scores (2-sample t-test)

Comparing Two SUS Scores (2-sample t-test)

Computing Sample Sizes

Computing Sample Sizes

All Positive SUS Scoring Sheet

All Positive SUS Scoring Sheet

Sample Page 23

Sample Page 23

Sample Page 24

Sample Page 24

Sample page 65

Sample page 65

Sample Page 119

Sample Page 119

The 150 Page Practical Guide to SUS

The 150 Page Practical Guide to SUS

The guide provides step-by-step instructions for using the calculator as well as providing example calculations, benchmarks and the latest research on SUS from over 5000 responses.

Highlights

  • Prevents Improper Coding
  • Provides background and best-practices and benchmarks
  • Compares SUS Scores Statistically
  • Handles Missing Values
  • Computes Sample Sizes
  • Checks for Inconsistent Respondents
  • Converts SUS scores to Percentile Ranks
  • Converts SUS scores to Grades (A+ to F)
  • Discussion on SUS Reliability & Validity
  • Understand what can and can't be modified on the SUS
  • Checks Internal Reliability

What Customers are Saying:

This is a great piece of work that will be useful to a lot of people.

John Brooke PhD
System Usability Scale Creator
The SUS calculator and the guide are terrific. I'm not a statistician so tools like this are a God send for me.

Randall Blair
Usability Engineer
Imodules
Definitely a great toolkit for calculating the SUS. Well worth the purchase. And the best part, was the customer service!

Amy Coffey
Business Consultant
A model of lucidity and an indispensable handbook (in the true sense of the word) for using the SUS questionnaire.

Jurek Kirakowski Ph.D.
Human Factors Research Group,
University College Cork, Ireland


     

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