User Interface Design - Requirements

Requirements

The dynamic characteristics of a system are described in terms of the dialogue requirements contained in seven principles of part 10 of the ergonomics standard, the ISO 9241. This standard establishes a framework of ergonomic "principles" for the dialogue techniques with high-level definitions and illustrative applications and examples of the principles. The principles of the dialogue represent the dynamic aspects of the interface and can be mostly regarded as the "feel" of the interface. The seven dialogue principles are:

• Suitability for the task: the dialogue is suitable for a task when it supports the user in the effective and efficient completion of the task.
• Self-descriptiveness: the dialogue is self-descriptive when each dialogue step is immediately comprehensible through feedback from the system or is explained to the user on request.
• Controllability: the dialogue is controllable when the user is able to initiate and control the direction and pace of the interaction until the point at which the goal has been met.
• Conformity with user expectations: the dialogue conforms with user expectations when it is consistent and corresponds to the user characteristics, such as task knowledge, education, experience, and to commonly accepted conventions.
• Error tolerance: the dialogue is error tolerant if despite evident errors in input, the intended result may be achieved with either no or minimal action by the user.
• Suitability for individualization: the dialogue is capable of individualization when the interface software can be modified to suit the task needs, individual preferences, and skills of the user.
• Suitability for learning: the dialogue is suitable for learning when it supports and guides the user in learning to use the system.

The concept of usability is defined of the ISO 9241 standard by effectiveness, efficiency, and satisfaction of the user. Part 11 gives the following definition of usability:

• Usability is measured by the extent to which the intended goals of use of the overall system are achieved (effectiveness).
• The resources that have to be expended to achieve the intended goals (efficiency).
• The extent to which the user finds the overall system acceptable (satisfaction).

Effectiveness, efficiency, and satisfaction can be seen as quality factors of usability. To evaluate these factors, they need to be decomposed into sub-factors, and finally, into usability measures.

The information presentation is described in Part 12 of the ISO 9241 standard for the organization of information (arrangement, alignment, grouping, labels, location), for the display of graphical objects, and for the coding of information (abbreviation, color, size, shape, visual cues) by seven attributes. The "attributes of presented information" represent the static aspects of the interface and can be generally regarded as the "look" of the interface. The attributes are detailed in the recommendations given in the standard. Each of the recommendations supports one or more of the seven attributes. The seven presentation attributes are:

• Clarity: the information content is conveyed quickly and accurately.
• Discriminability: the displayed information can be distinguished accurately.
• Conciseness: users are not overloaded with extraneous information.
• Consistency: a unique design, conformity with user’s expectation.
• Detectability: the user’s attention is directed towards information required.
• Legibility: information is easy to read.
• Comprehensibility: the meaning is clearly understandable, unambiguous, interpretable, and recognizable.

The user guidance in Part 13 of the ISO 9241 standard describes that the user guidance information should be readily distinguishable from other displayed information and should be specific for the current context of use. User guidance can be given by the following five means:

• Prompts indicating explicitly (specific prompts) or implicitly (generic prompts) that the system is available for input.
• Feedback informing about the user’s input timely, perceptible, and non-intrusive.
• Status information indicating the continuing state of the application, the system’s hardware and software components, and the user’s activities.
• Error management including error prevention, error correction, user support for error management, and error messages.
• On-line help for system-initiated and user initiated requests with specific information for the current context of use.