If usability can be defined by a single, standardized usability metric it would me much easier for it to be measured and managed and to be put on the agenda of boardroom meetings. Defining usability in a single metric would make it more accessible to managers. Is it possible though?
However there are several definitions of what usability is. Schneiderman for example identify five usability measures: time to learn, speed of performance, rate of errors by users, retention over time and subjective satisfaction. According to ISO-9241-11 usability consists of the three aspects: effectiveness, efficiency and subjective satisfaction and Jakob Nielsen defines usability by the five following components: learnability, efficiency, memorability, errors and satisfaction.
Some researcher like McGee and Sauro and Kindlund believe that measuring usability in a single, standardizd metric is possible. They think that the different aspects of usability to some degree contribute the same information and can be combined.
To find out if such a strong correlation between the different aspects of usability exists Hornbaek and Lai-Chong Law performed a Meta-Analysis of Correlations Among Usability Measures.
Altogether 73 datasets were analyzed. Their study shows that there is a small to medium correlation between typical measures of usability. This makes defining usability in a single, standardized metric extremely difficult. Until the holy grail has been found it is better to use one of the defintions as stated by Schneiderman, ISO-9241-11 or Nielsen.
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