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|Contrary to popular belief Usability is not some objective property of a product.
ISO 9241-11 defines Usability as: "Extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use."
To break this down:
"Extent to wich a product can be used..."
It is a somewhat measurable property. It may not be easily quantifiable but can and should be subject of qualitative analysis.
"...by specified users..."
It's impossible for a product to usable for all users (well almost). Why you should define exactly what user the product is supposed to be usable for, to be able to determine if that is true.
"Joe Sixpack is the generic guy who sits on his couch watching the football, hockey, soccer, baseball games incessantly, in between watching trashy american sitcoms. This person believes he's a great person and citizen if he votes once every four years." - [http://www.everything2.com/index.pl?node_id=152555 Joe Sixpack at everything2.com]
Although it can be argued that JoeSixpack isn't a very good persona. He is to generic and undefined. A good persona is the result of carful analysis of a specific target group.
"...to achieve specified goals..."
Just as a product must be designed for a specific user it also has to be designed to be usable to achieve a specific goal. It can't be just "usable" it must be usable for something.
A common mistake is to define goals as the technical effect of using a product. A goal for which a product is usable is almost certainly external to the product. Most importantly the goal is something that is important to the specified user, it is not generic.
A goal such as compress sound files is probably to techincal, the goal might be to store music or something along those lines. It depends on the user and one of the hard part of usability analysis. Usually the user isn't very good at expressing his/her goals so carful analysis is required to determine usefull goals.
"...with effectiveness, efficiency and satisfaction..."
Also a commin misconception is that usability is isolated to efficiency or effectiveness, or even worse ease of use. Satisfaction is an important part of usability, becase this is what makes a user want to use the product.
It's only logical that a usable product actually have to be used. A common metric when designing for usability is usage rate, the extent to which the product is actually used in the day to day lives of the users.
"..in a specified context of use."
An often missed part of usability is the context. When and where and under what conditions is the product to be used?
The obvious example is a paper and a pencil. If you study this simple product you'll probably find that it's exceptionally usable. But would be utterly useless if the specified context of use is: underwater at a dive.