Self-Reported Metrics - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 146

may be that it doesn't try to break down the assessment into more detailed compo-
nents (e.g., ease of learning, ease of navigation). All 10 of the rating scales in SUS are
simply asking for an assessment of the site as a whole, just in slightly different ways.

6.4.8 Net Promoter Score
One self-reported metric that has gained rapidly in popularity, especially among
senior executives, is the Net Promoter Score (NPS). It's intended to be a measure
of customer loyalty, and was originated by Fred Reichheld in his 2003 article in the
Harvard Business Review : “One Number You Need to Grow” (Reichheld, 2003). The
power of NPS seems to derive from its simplicity, as it uses only one question: “How
likely is it that you would recommend [this company, product, website, etc] to a friend
or colleague?” The respondent answers using an 11-point scale of 0 (Not at all likely)
to 10 (Extremely likely). The respondents are then divided into three categories:

• Detractors:Thosewhogaveratingsof0-6

• Passives:Thosewhogaveratingsof7or8

• Promoters:Thosewhogaveratingsof9or10

Note that the categorization into Detractors, Passives, and Promoters is nowhere
near symmetrical. By design, the bar is set pretty high to be a Promoter, while it's
very easy to be a Detractor. To calculate the NPS, you subtract the percentage of
Detractors (ratings of 0-6) from the percentage of Promoters (ratings of 9 or 10).
Passives are ignored in the calculation. In theory, NPSs can range from −100 to +100.

The NPS is not without its own detractors. One criticism is that the reduction
of scores from an 11-point scale to just three categories (Detractors, Passives,
Promoters) results in a loss of statistical power and precision. This is similar
to the loss of precision when using the “Top Box” or “Top-2-Box” method of
analysis discussed earlier in this chapter. But you lose even more precision when
you take the difference between two percentages (Promoters minus Detractors),
which is similar to subtracting “Bottom Box” scores from “Top Box” scores. Each
percentage (% Promoters and % Detractors) has its own confidence interval (or
margin of error) associated with it. The confidence interval associated with the
difference between the two percentages is essentially the combination of the two
individual confidence intervals. You would typically need a sample size two to
four times larger to get an NPS margin of error equivalent to the margin of error
for a traditional Top-2-Box score. Case Study 10.1 provides an excellent example
of how NPS can be used to improve the user experience.

DOES PERCEIVED USABILITY PREDICT CUSTOMER LOYALTY?

Jeff Sauro (2010) wanted to know whether usability, as measured by SUS, tended to
predict customer loyalty, as measured by NPS. He analyzed data from 146 users asked
to complete both the SUS questions and the NPS question for a variety of products,
including websites and financial applications. The result was a correlation of r = 0.61,
which is highly significant ( p < 0.001). He found that Promoters had an average SUS
score of 82, while Detractors had an average SUS score of 67.