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:
- characteristics of the computer system (user interface features including system functionality)
- user features (knowledge, abilities, skills, motivations, attitude towards the activity and the system)
- responsibilities and job characteristics (type, difficulty and task width; task variety; self-reliance and freedom of choice during the task processing; terms of the user education and support; availability of the system to the user; task-related frequency of system use)
The system must be useful to be used, which depends on functionality, usability and reliability. It has to please the user in some way.