Usability - what is it anyway?

You encounter usability (or rather lack of it) every time you try to do something and end up pulling your hair from frustration when your tools (either software or physical device) seems to do everything in its power to hinder you work, but what exactly is usability? There are almost as many definitions for usability as there are researchers trying to define it. Usability is one of the main software product and system quality attributes described in the international standard ISO 9126 where usability is linked to the capability of the product to be understood by, learned, used by and attractive to the specified user, when used under specified conditions. Another common definition of usability is in standard ISO 9241-11, where usability is defined as: The 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.

Nielsen (1993) and Shneiderman (1998) have both independently defined five key attributes of usability that applies to all aspects of a system with human-computer interaction. The key usability attributes are according to Nielsen/Schneiderman: learnability/ease of learning, efficiency/speed of performance, low rate and severity of errors, memorability/retention over time, and subjective user satisfaction/user attitude. How high would you rank the tools you have to work with using these five key usability attributes?