Setting Usability Criteria

Measurable usability criteria for the user interface should be used to assess the likely success of the system during testing or user trials.

Usability criteria to consider:

How easy do users find it to learn to use the interface?
How efficient are users once they are familiar with the interface? (Productivity should increase.)
How easy do occasional users find it to re-learn how to use the system after a break? Good design features, such as keeping the number of menu levels low, should help. Keeping a record of recurring problems will also be helpful, possibly even using the system itself to record problems.
How often do errors occur and how serious are they? Do any errors occur repeatedly? These criteria can be measured by counting and classifying the errors to give the number of errors as a percentage of the total number of times that a procedure was used. This will help to balance error ratios for often and infrequently used procedures.
How satisfied are the users? This can be subjective, but it does help to gauge user acceptance of the interface. However, the speed with which new users take up the new system or facilities may provide some measurable indication of satisfaction.