Issue-Based Metrics - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 105

AN EXAMPLE OF THE ULTIMATE ISSUE SEVERITY
Tullis (2011) described an example of what we consider the ultimate in issue severity.
In the early 1980s he conducted a usability test of a prototype of a handheld device for
detecting high voltage on a metallic surface. The device had two indicator lights: one
simply indicated that the device is working and the other indicated that there is high
voltage present, which could be fatal. Unfortunately, both indicator lights were green.
And they were right next to each other. And neither was labeled. After pleading with the
designers to change the design, he finally decided to do a quick usability test. He had
10 participants perform 10 simulated tasks with the device. The prototype was rigged to
signal the hazardous voltage condition 20% of the time. Out of 100 participant tasks,
theindicatorlightswereinterpretedcorrectly99times.Butthatoneerrorwaswhen
it was signaling hazardous voltage. This usability issue could have resulted in serious
injury or death to the user. The designers were convinced and the design was changed
significantly.
5.3.2 Severity Ratings Based on a Combination of Factors
Severity rating systems that use a combination of factors usually are based on the
impactontheuserexperiencecoupledwithfrequencyofuseand/orimpacton
thebusinessgoals.Nielsen(1993)providesaneasywaytocombinetheimpact
on the user experience and frequency of use on severity ratings ( Figure 5.1 ). This
severity rating system is intuitive and easy to explain.