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:
- The ease/speed of learning to use the system for given task width and variety
- The ease and efficiency of system usage that shows in performance variables like speed, task processing accuracy, utilization of the system functionality, stress of using the system during the task processing
- Indicators of acceptance: subjective (user's attitude and reviews of the system regarding how easy, interesting and advantageous it is to process tasks with the system), objective (frequency of voluntary system usage, type of tasks that users freely decide to process)
- The voluntary, active effort of the user to learn about the system and how to use it
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.