What is Usability ?

Usability is not a single term, but a combination of many principles that apply to a software application. It questions whether that application is usable by common users. Whether users understand it ?. A few definition are:

a) Learnability -
> How quickly can a novice start using the product?
> How quickly can a novice become an advanced beginner and an advanced beginner become a Competent performer?
> How much of training and practice is necessary to become a user at a given stage?
> How many novice users and advanced beginners explore the software to become more efficient / ...advanced in stages?

b) Ease of Use -
> How easy is it to use the product for users at a given stage?
> How many users in different stages can use the product, and to what level?
> How many tasks do users at a given stage need to do that they need not or should not do?
> How much information do the users need to provide that the computer already has or should have?
> How easy is it for a group of users to work together?

c) Error - Free Use -
> How many and what kind of errors (catastrophic, major and minor) are made by users at different stages
... to carry out benchmark tasks?
> How many errors do users make repeatedly?
> How many times do users do different things than what they intended because they misunderstood?
> How long does it take for the users to identify the error condition?
> How many users can identify the method of recovering from errors?
> How forgiving is the interface? What percentage of errors can users recover from?
> How much time does it take for users to recover from errors?

d) Speed of Use -
> How long does it take for the user at a given stage to carry out a benchmark set of tasks?
> How long does it take for users to recover from errors?

e) Retention over Time -
> How much of the interface can a user at given stage recall after predetermined gaps (hours, days, ...months, years)?
> With how much effort and time can a user at a given stage relearn what he has forgotten?

f) Subjective Satisfaction -
> How much did the users like the software?
> For users at given stages, how does it compare over competition (including doing things manually)?
> What kind of experience does the product give to its users?


.