Learnability
How easy is it for users to accomplish basic tasks the first time they encounter the design? Efficiency
Once users have learned the design, how quickly can they perform tasks?Memorability
When users return to the design after a period of not using it, how easily can they reestablish proficiency?Errors
How many errors do users make, how severe are these errors, and how easily can they recover from the errors?Satisfaction
How pleasant is it to use the design?