ANSWER

Usefulness

Ensure that the system includes the most important features and functions needed to provide value to the user.  A system that is missing high priority functionality is not going to be very valuable.  Similarly, a system that includes too much additional functionality which lacks relevance to the user will obfuscate the intended flow through the system.

Consistency

Consistently apply the UI design patterns that your group has agreed upon and adopted ahead of time. This includes UI actions, product terminology and UI commands. UI design pattern consistency is important across screens within an application as well as across applications within a suite of products.  Whenever appropriate, the concepts, terms and metaphors used should mirror that of the real world and present information in a logical order.
 
Simplicity

Keep the design clean and clutter free eliminating any unnecessary or irrelevant information, features, controls, or icons. When many options are available for a user to complete their task, keep the most commonly used options visible and make the other options readily accessible via some other means.  Design the system with the goal of being intuitive enough that an appropriate user of the system can learn it without instruction or formal training. 

Communication and Feedback

The system should provide clear and timely feedback in response to user actions.  This reinforces within the user’s mind that they have taken the correct step to successfully complete the task. Organize and sequence the information within a screen in a way that conveys specific meaning to the user.  Like information should be grouped together and the order in which a user encounters information should reinforce the flow of the process as they currently understand it.  Ensure that whatever help information is provided is concise and focuses specifically on the user’s task.

Error Prevention and Handling

A system cannot and should not catch all mistakes.  It should catch serious errors when possible, but should not be so restrictive that it does not allow for reasonable variation in data entry or process flow.  Ask the user for confirmation before allowing a potentially destructive action such as permanently deleting a record.  If the user performs an incorrect action, provide clear and descriptive messaging that describes what they have done wrong and when possible how they can correct it. To relieve anxiety, provide the user with the ability to discard unwanted changes, or to undo a particularly sensitive action.

Efficiency

Ensure that skilled users can use the system efficiently and that functionality built for inexperienced users does not slow down skilled users. Provide shortcuts keys and functions for the experienced user and allow for customization of shortcuts when possible for frequent actions.

Workload Reduction

Automate aspects of the process and system whenever possible.  If something can be easily derived by the application then don’t make the user spend time doing it.  Eliminate mental calculations and comparisons by showing the results on screen. Ensure that users don’t need to remember information from one screen to another.  Provide the necessary information on each screen for them to be able to complete their work.

Designer Judgment

Remember that usability guidelines are just that, guidelines. There are tradeoffs between perfect UI design, user experience, and system performance. There will always be cases where it makes send to not follow a particular guideline or principle.  Use your judgment. However, try to stray from guidelines as infrequently as possible and, when doing so, be sure to document the reason why the guideline was not followed for future reference.