To deliver a truly usable and effective solution a number of principles should be included in the design of any interface and user interaction. These should then be tested with the end user audience to ensure they are effective.
Heuristic definition: “Encouraging a person to learn, discover, understand, or solve problems on his or her own, by experimenting, evaluating possible answers or solutions, or by trial and error.”
Nielsen’s Heuristics: Rolf Molich and Jakob Nielsen (1990) developed a set of heuristics that are probably the most used in the field of interface design. Nielsen later (1994), revised these to the following set:
Visibility of system status
The system should always keep users informed about what is going on, through appropriate feedback and within a reasonable time.
Match between system and the real world
The system should speak the users' language, with words, phrases and concepts familiar to the user, rather than system-oriented terms. Follow real-world conventions, making information appear in a natural and logical order.
User control and freedom
Users often choose system functions by mistake and will need a clearly marked "emergency exit" to leave the unwanted state or place in a system without having to go through an extended dialogue or series of steps. Support undo and redo.
Consistency and standards
Users should not have to wonder whether different words, situations, or actions mean the same thing. Follow standardised terminology and conventions.
Even better than good error messages is a careful design which prevents a problem from occurring in the first place. Either eliminate error-prone actions or check for them and present users with a confirmation option before they commit to the action.
Recognition rather than recall
Minimize the user's memory load by making objects, actions, and options visible. The user should not have to remember information from one part of the system to another. Instructions for use of the system should be visible or easily retrievable whenever appropriate.
Flexibility and efficiency of use
Accelerators -- unseen by the novice user -- may often speed up the interaction for the expert user such that the system can cater to both inexperienced and experienced users. Allow users to tailor frequent actions.
Aesthetic and minimalist design
Areas should not contain information which is irrelevant or rarely needed. Every extra unit of information in an area competes with the relevant units of information and diminishes their relative visibility.
Help users recognize, diagnose, and recover from errors
Error messages should be expressed in plain language (no codes), precisely indicate the problem, and constructively suggest a solution.
Help and documentation
Even though it is better if the system can be used without documentation, it may be necessary to provide help and documentation. Any such information should be easy to search, focused on the users’ tasks, list concrete steps to be carried out, and should not be too large.