Usability

Definition

The utility quality subfactor, usability, is the ease with which a specific set of users is able to effectively use an application or component (e.g., user interface, help facilities) and its documentation (e.g., error messages, user manual).

Types

The utility quality subfactor can be decomposed into the following subfactors:

• Attractiveness (a.k.a., engagability, stickyness).
  The degree to which users find something 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., trustworthyness).
  The degree to which users are confident with and have trust in something including that its:
  • Output and behavior are correct.
  • Content is authoritative.
  • Owner’s motives are trustworthy.
  • Developers’ are competent.
• Differentiation.
  The degree to which something differentiates itself from its competition.
• Ease to Entry.
  The ease with which users can start using the application
  (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 to Learning.
  The degree to which representatives users can learn to use it to perform their tasks.
• Ease to Location.
  The ease with which users can find the application or document (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 something to perform common tasks or the degree to which regular users to can remember how to use something to perform infrequent tasks.
• Ease to Use.
  The ease with which users can use something to perform their tasks.
• Effectiveness (a.k.a., operability).
  The degree to which the something enables users to efficiently perform their tasks.
• Error Minimization.
  The degree to which something minimizes the number of errors that its users make.
• Navigability.
  The degree to which users can move through the user interface or documentation to find desired content and to perform their tasks.
• Preferance.
  The degree to which users prefer something over its alternatives.
• Retrievability.
  The ease with which an application 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 something to be suitable for the performance of their tasks.
• Understandability.
  The degree to which users find something to be clear, legible, unambiguous, and comprehensible (especially during unusual situations).
• User-Satisfaction.
  The degree to which users are satisfied with something and consider it to be beneficial to them.

Guidelines

The following guidelines have been found to be useful when producing usability requirements:

• 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).