Usability

Definition

Usability

the utility quality factor representing the degree to which a product’s human user interface enables a specified group of users achieve specified goals in a specified context of use

Classification

As illustrated in the preceding figure, Robustness is part of the following inheritance hierarchy:

• Type: Abstract
• Superclass: Utility
• Subclasses:
  • Data Usability
  • Hardware Usability
  • Software Usability

Responsibilities

The typical responsibilities of Usability are to:

• Model the degree to which a product’s human user interface enables a specified group of users achieve specified goals in a specified context of use.
• Support the analysis and specification of usability requirements.
• Provide a foundation for evaluating the quality of an architecture.

Subfactors

As illustrated in the following graphic, usability can be decomposed into the following aggregation hierarchy of subfactors:

• Attractiveness (a.k.a., engagability, preference, stickiness)
  The degree to which users find the product to:
  • Be attractive or appealing
  • Engage their attention
  • Provide a positive user experience
  • Make them continue to use it
  • Make them return to use it in the future
• Credibility (a.k.a., trustworthiness)
  The degree to which users are confident with and have trust in the product including that its:
  • Output and behavior are correct
  • Content is authoritative
  • Owner’s motives are trustworthy
  • Developers are competent
• Differentiation
  The degree to which the product differentiates itself from competing products
• Ease of Entry
  The ease with which users can start using the product
  (e.g., can log on and begin using their desired functionality without taking forever to be identified, authenticated, and navigate to the point where they can start performing their tasks)
• Ease of Learning
  The degree to which representatives users can learn to use the product to achieve their goals (e.g., to find desired content and to perform their tasks)
• Ease of Location
  The ease with which users can find the product’s content or services (e.g., finding Web applications such as websites using search engines)
• Ease of Remembering
  Either the degree to which occasional users can remember how to use the product to perform common tasks or the degree to which regular users to can remember how to use the product to perform infrequent tasks
• Effectiveness (a.k.a., operability)
  The degree to which the product its enables users to successfully achieve their goals
• Efficiency
  The degree to which the product minimizes the resources expended in relation to the accuracy and completeness with which users achieve their goals
• Error Minimization
  The degree to which the product minimizes the number of errors that its users make
• Navigability
  The degree to which the product enables users to easily move through the user interface or documentation to achieve their goals
• Preference
  The degree to which users prefer the product over its alternatives
• Retrievability
  The ease with which the product enables users to obtain information in a form that is useful to them (e.g., print out a paper report, make a copy of a multimedia file)
• Suitability
  The degree to which users find that the product to be suitable for the performance of their tasks
• Understandability
  The degree to which users find the product’s user interface and output to be clear, legible, unambiguous, and comprehensible (especially during unusual situations)
• User-Satisfaction
  The degree to which users are satisfied with the product and consider it to be beneficial to them

Measures

Usability is typically measured in terms of the:

• TBD

Mechanisms

Typical mechanisms for implementing usability include:

• TBD

Guidelines

The following guidelines have been found to be useful with regard to usability:

• When usability requirements are specified as the desires and needs of the customer organization and user organizations, the tend to be at ambiguous and not validatable and therefore inadequate for their other stakeholders, especially the architecture team and the user experience team that must perform system usability testing. On the other hand, if the usability requirements are specified in the necessary detail to make them truly unambiguous and validatable, then the tend to be so long and complex that they are difficult for the customer representatives and user representatives to understand. Thus, usability requirements should typically be specified in two parts:
  • A brief potentially ambiguous statement of the intent of the requirement that is easy for the customer and user representatives to read, understand, and approve.
  • A longer, complete requirement (the validation criteria) that provides the detail required by the architecture, development, and testing teams.
• By adding validation criteria to the basic usability requirements, a formal consensus is produced as to the exact meaning of the requirement and the effort to produce usability test cases is minimized.
• Usability is typically specified and measured in terms of the:
  • Amount of time it takes for different types of users to learn to effectively use the application or component.
  • Productivity of the different types of users when they perform their tasks.
  • Number and types of errors (and associated defects) that the different types of users make (and introduce).
• Learnability and ease of remembering are important for functions that are infrequently performed and thus may be forgotten between being performed.
• Error minimizing is related to robustness in that error minimizing prevents human errors from being made whereas robustness either prevents or properly handles failures caused by defects that in turn were caused by human errors.
• Usability testing is typically used to iteratively and incrementally validate the usability requirements.
• Usability failures found during usability testing can be classified by severity:
  • Total Failure. Either the user cannot complete the task on his or her own, or else erroneously believes that the task has been completed.
  • Major Failure. Although the user successfully completes the task, the user also complains that the application is annoying or difficult to use.
  • Medium Failure. Although the user does not complain, the user nevertheless requires multiple attempts to successfully complete the task.
  • Minor Failure. The user requires a few short attempts to successfully complete the task.
• Usability depends greatly on the:
  • Type of user.
  • User’s training.
  • User’s experience level.
• The scope of product usability should include the application or component’s:
  • User interface.
  • Help facilities.
  • User documentation.
• Usability requirements are related to (and should not redundantly specify) other quality requirements.
  • An application or component is not usable if it is so inefficient (i.e., efficiency requirements) that its performance (i.e., response time and throughput requirements) prevents the user from using it.
  • Personalization makes a user interface more engaging (i.e., engagability requirements).