Heuristic Evaluation - Cross-Cultural Human-Computer Interaction and User Experience Design - page 165

Perceived stability.
In order to cope with the new level of complexity that computers introduce, people
need stable reference points. To give users a conceptual sense of stability, the interface
provides a clear finite set of objects with a clear, finite set of actions. When particular
actions are unavailable, they are not eliminated, but are dimmed.
Recognition rather than recall.
Make objects, actions, and options visible. The user should not have to remember
information from one part of the dialogue to another. Instructions for use of the
system should be visible or easily retrievable whenever appropriate.
User control and freedom.
Allow the user, not the computer to initiate and control actions. Users often choose
system functions by mistake and will need a clearly marked “emergency exit” to leave
the unwanted state without having to go through an extended dialogue. Support undo
and redo functions.
Visible interfaces/WYSIWYG.
Don't hide features in your application by using abstract commands. People should
be able to see what they need when they need it. Most users cannot and will not build
elaborate mental maps and will become lost or tired if expected to do so.
Severity ratings.
The severity of a usability problem is a combination of three factors:
1. The frequency with which the problem occurs: Is it common or rare?
2. The impact of the problem if it occurs: Will it be easy or difficult for the
users to overcome?
3. The persistence of the problem: Is it a one-time problem that users can
overcome once they know about it or will users repeatedly be bothered by
the problem?
Finally, of course, one needs to assess the market impact of the problem since
certain usability problems can have a devastating effect on the popularity of a product,
even if they are “objectively” quite easy to overcome. Even though severity has several
components, it is common to combine all aspects of severity in a single severity rating
as an overall assessment of each usability problem in order to facilitate prioritizing
and decision making.
The severity ratings used in this report are described below:
• Severity level 1: Cosmetic problem only—need not be fixed unless extra
time is available on the project.
• Severity level 2: Minor usability problem—could impair users' productivity
and ability to learn.