Table 2.

Usability Factors and Empirical Measurements ²⁰, ⁴⁰-⁴²

Factors	Objective Measures	Subjective Measures
Learnability: ease with which use of device is learned so users can rapidly accomplish intended tasks	Clock time for new users to learn to accomplish the intended tasks	Users’ ratings of the ease and time to learn the system
Effectiveness: usefulness for supporting intended tasks	Successful performance of the intended tasks; measures of productivity	Users’ ratings of the system’s ability to promote their performance and productivity
Efficiency: productivity once users have learned the system	Clock time to accomplish the tasks once users have learned the system	Users’ ratings of the system’s ability to improve the speed at which they perform
Errors: low frequency, severity of errors and easy recovery	Error rates trying to use system; Severity of errors; Recovery time for errors	Users’ ratings of the impact of errors on using system and their ability to recover from errors
Flexibility: variety of ways to achieve intended tasks	Number of different commands or routes to achieve the same goal	Users’ ratings of system’s ability to provide different commands or routes to achieve the same goal
Memorability: ease with which casual users can return to the system without having to relearn	Memory failure rate on how to use system the next time; time to re-learn the system after periods of non-use	Users’ ratings of the ability to remember how to use the system the next time and their ability to re-learn the system after periods of non-use
User Satisfaction: pleasant for users	Frequency for utilization of the system and its features	Users’ ratings of their fondness, perceptions and opinions of the system and its features