Self-Reported Metrics - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 139

reported SUS scores for many different products and systems, including desktop
applications, websites, voice-response systems, and various consumer products.
Tullis (2008) and Bangor, Kortum, and Miller (2009) both reported analyses of
SUS scores from a wide variety of studies. Tullis (2008a) reported data from 129
different uses of SUS, while Bangor and colleagues (2009) reported data from
206. Frequency distributions of the two sets of data are remarkably similar, as
shown in Figure 6.9 , with a median study score of 69 for Tullis data and 71 for
Bangor et al. data. Bangor and colleagues suggested the following interpretation
of SUS scores based on their data:

• <50: Not acceptable

• 50-70:Marginal

• >70: Acceptable

FACTORS IN SUS

Although SUS was originally designed to assess perceived usability as a single attribute,
Lewis and Sauro (2009) found that there are actually two factors in SUS. Eight of
the questions reflect a usability factor and two reflect a learnability factor. It's easy to
compute both from raw SUS ratings.

Frequency Distributions of Mean SUS Scores

12%

Tullis (2008)
Bangor et al (2009)

10%

8%

6%

4%

2%

0%

0510 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Figure 6.9 Frequency distributions of mean SUS scores reported by Tullis (2008a) and by Bangor et al.
(2009). Tullis data are based on a total of 129 study conditions, and Bangor et al. data are based on 206.