Usability variables

In 1993, Jens Wandmacher has published a book on software ergonomy (has a free preview on Google Books), where he defined some usability variables in a way that can still be useful today. He distinguishes between dependent and independent usability variables.

Dependent usability variables are:

Independent usability variables are:

The system must be useful to be used, which depends on functionality, usability and reliability. It has to please the user in some way.