Operational criteria for usability

From Shackle(1986):

Usability can be specified and measured via the operational criteria defined below. The terms should be given numerical values when the usability goals are set during the design stage of 'requirements specification'.

For a system to be usable the following must be achieved-
The range of required tasks must be accomplished-

EFFECTIVENESS

• at better than some required level of performance (e.g., in terms of speed and errors)
• by some required percentage of the specified target range of users
• within some required proportion of the range of usage environments

LEARNABILITY

• within some specified time from installation and start of user training
• based upon some specified amount of training and user support
• and within some specified relearning time each time for intermittent users

FLEXIBILITY

• with flexibility allowing adaptation to some specified percentage variation in tasks and/or environments beyond those first specified

ATTITUDE

• and within acceptable levels of human cost in terms of tiredness, discomfort, frustration and personal effort

ISO definition of usability:

"The usability of a product is the degree to which specific users can achieve specific goals within a particular environment; effectively, efficiently, comfortably, and in an acceptable manner."

ISO 9241 provides a highly detailed criteria for this broad definition