Usability Requirements

Definition

A usability requirement is a utility requirement that specifies a required amount of usability, which is a quality factor that is defined as follows:

Usability

adj.[quality factor] ease with which a specified set of users are able to effectively use something.

Objectives

The typical objectives of a usability requirement are to ensure that an application, component, and its documentation have the required amounts of the following subcategories of the usability quality factor:

• 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.

Examples

The following are typical examples of usability requirements (including associated validation criteria), listed in order of the preceding quality subfactors:

• Attractiveness (a.k.a., engagability, stickyness).
  
  • “The application’s user interface shall be attractive to its users.
    Specifically, at least 90% of a statistically valid sample of users shall rate the overall user interface as being either very attractive or attractive on the following scale (very attractive, attractive, neutral, ugly, very ugly).”
  • “The application shall provide a positive experience to its users.
    Specifically, at least 90% of a statistically valid sample of users shall rate the overall user experience as being either very positive or negative on the following scale (very positive, positive, neutral, negative, very negative).”
  • “The application’s user interface shall engage its users attention.
    Specifically, at least 90% of a statistically valid sample of users shall rate the overall user interface as being either very engaging or engaging on the following scale (very engaging, engaging, neutral, boring, very boring).”
  • “The application shall be sticky.
    Specifically, at least 90% of the uses of the application by a statistically valid sample of users shall last at least 15 minutes before exiting or starting to use another application.”
  • “The application shall encourage repeat usage.
    Specifically, at least 80% of a statistically valid sample of users shall continue to use it after an initial 4-week trail usage period.”
• Credibility (a.k.a., trustworthyness).
  
  • “The application shall elicit a feeling of trustworthyness in its users.
    Specifically, at least 90% of a statistically valid sample of users shall rate the application as being either very trustworthy or trustworthy on the following scale (very trustworthy, trustworthy, neutral, untrustworthy, very untrustworthy).”
  • “The application shall evoke in its users the feeling that its output is trustworthy.
    Specifically, at least 95% of a statistically valid sample of users shall rate the application’s output as being either highly correct or mostly correct on the following scale (highly correct, mostly correct, neutral, mostly incorrect, highly incorrect).”
  • “The informational website shall evoke in its users the feeling that its content is authoritative.
    Specifically, at least 95% of a statistically valid sample of users shall rate the website’s content as being either very authoritative or authoritative on the following scale (totally authoritative, somewhat authoritative, neutral, somewhat not authoritative, totally not authoritative).”
  • “The application shall evoke in its users the feeling of trust in the its owner’s business.
    Specifically, at least 80% of a statistically valid sample of users shall rate the business of the application’s owners as being either very trustworthy or trustworthy on the following scale (very trustworthy, trustworthy, neutral, untrustworthy, very untrustworthy).”
  • “The application shall evoke in its users the feeling of trust in the its developer’s competence.
    Specifically, at least 75% of a statistically valid sample of users shall rate the developers of the application as being either very competent or somewhat competent on the following scale (very competent, somewhat competent, neutral, somewhat incompetent, very incompetent).”
• Differentiation.
  
  • “The application shall stand out when compared with its competing applications.
    Specifically, at least 90% of a statistically valid sample of users shall rate the application as being either unique or trustworthy on the following scale (unique, stands out, neutral, mostly the same, the same as the rest).”
• Ease to Entry.
  
  • “The application shall be easy for its users to begin using.
    Specifically, at least 90% of a statistically valid sample of users shall rate the application as being either very easy to begin using or easy to begin using on the following scale (very easy to begin using, easy to begin using, neutral, difficult to begin using, very difficult to begin using).”
  • “The application shall not require its users to perform numerous steps before they can begin using it.
    Specifically, users shall be able to begin using the application to perform their tasks within 2 minutes starting at least 90% of the time.”
• Ease to Learning.
  
  • “The application shall be easy for its users to learn how to use to perform their tasks.
    Specifically, at least 90% of a statistically valid sample of users shall rate the application as being either very easy to learn or easy to learn on the following scale (very easy to learn, easy to learn, neutral, difficult to learn, very difficult to learn).”
  • “The application shall not require users to take significant training to learn how to use it to perform their tasks.
    Specifically, the application shall enable at least 90% of a statistically valid sample of users to successfully perform their assigned tasks after receiving no more than 2 hours of instruction.”
  • “The application shall enable the general public to perform X without prior training.
    Specifically, the application shall enable at least 90% of a statistically valid sample of the general public to use the application to perform task X in under 5 minutes without needing prior training.”
• Ease to Location.
  
  • “The application shall be easy for its users to locate.
    Specifically, at least 90% of a statistically valid sample of users shall rate the application as being either very easy to locate or easy to locate on the following scale (very easy to locate, easy to locate, neutral, difficult to locate, very difficult to locate).”
  • “The application’s help facilities shall be easy to locate.
    Specifically, at least 90% of a statistically valid sample of users shall rate the application’s on-line help facilities as being either very easy to locate or easy to locate on the following scale (very easy to locate, easy to locate, neutral, difficult to locate, very difficult to locate).”
  • “The website shall be easy to locate on the Web using .
    Specifically, the application shall enable at least 90% of a statistically valid sample of its users to find and get to it on the corporate intranet within 3 minutes.”
• Ease of Remembering.
  
  • “The application shall be easy for its users to remember how to use.
    Specifically, at least 90% of a statistically valid sample of users shall rate the application as being either very easy to learn or easy to learn on the following scale (very easy to learn, easy to learn, neutral, difficult to learn, very difficult to learn).
  • “The application shall be easy for its users to remember how to use.
    Specifically, the application shall enable at least 90% of a statistically valid sample of trained users to successfully perform the following uncommon tasks.”
  • Specifically, the application shall enable at least 90% of a statistically valid sample of trained users to successfully perform the following common tasks after not having used the application for two weeks.”
  • “The application shall not require the user to unnecessarily remember previously entered data.
    Specifically, the application’s user interface shall not require the user to either remember or copy information from one screen/webpage to another. (For example, the application may automatically reenter previously entered data on the new page, or else it may provide drop-down lists that allow users to select and enter valid choices.)”
• Ease to Use.
  
  • “The application shall be easy for its users to use.
    Specifically, at least 90% of a statistically valid sample of users shall rate the application as being either very easy to use or easy to use on the following scale (very easy to use, easy to use, neutral, difficult to use, very difficult to use).
  • “The application shall make task X easy for its users to perform.
    Specifically, the application shall enable at least 90% of a statistically valid sample of users to successfully perform task X in under 5 minutes.
  • “The application shall make is easy for its users to perform their tasks without errors.
    Specifically, the application shall enable at least 90% of a statistically valid sample of users to successfully perform their assigned tasks with a human error rate of less than 2%.
  • “The application shall make is easy for its users to perform task Y without experiencing total or major usability failures.
    Specifically, the application shall enable at least 90% of a statistically valid sample of novice users to perform task Y without experiencing a ‘total’ usability failure or ‘major’ usability failure (as defined in the following guidelines) more than 5% of the time.
  • “The application shall make is easy for its users to perform task Z without experiencing many medium or minor usability failures.
    Specifically, the application shall enable at least 90% of a statistically valid sample of novice users to perform task Z without experiencing 2 or more ‘medium’ usability failures or 5 or more ‘minor’ failures (as defined in the following guidelines) more than 5% of the time.”
• Effectiveness (a.k.a., operability).
  
  • “The application shall improve the effectiveness of its users.
    Specifically, at least 90% of a statistically valid sample of users shall state that the application has made them either significantly more effective or more effective on the following scale (significantly more effective, more effective, neutral, less effective, significantly less effective).
  • “The application shall make its users more effective when performing their tasks.
    Specifically, the application shall enable at least 90% of a statistically valid sample of trained users to increase their productivity by 20% compared with using the existing system, whereby productivity is measured in terms of completed tasks per hour.”
  • “The application shall make its users more effective when performing task X.
    Specifically, the application shall enable at least 90% of a statistically valid sample of trained users to increase their productivity to 14 completions of task X per hour.”
  • “The application shall minimize the user’s required movements (e.g., keystrokes, mouse movements).
    Specifically, the user interface shall provide rapid ways for experienced users to perform common tasks. (For example, keyboard shortcuts or brief codes may be used to minimize keystrokes or mouse movements.)”
• Error Minimization.
  
  • “The application shall make is easy for its users to perform their tasks without errors.
    Specifically, the application shall enable at least 90% of a statistically valid sample of novice users to perform task Y without a single failure more than 5% of the time.”
  • “The application shall minimize the errors made by its users.
    Specifically, at least 90% of a statistically valid sample of users shall state that the application has made them either significantly less error prone or less error prone on the following scale (significantly less error prone, less error prone, neutral, more error prone, more error prone).”
  • “The application shall make is difficult for its users to introduce human error when perform task Y.
    Specifically, the application shall enable at least 90% of a statistically valid sample of novice users to perform task Y without making a human error more than 5% of the time.
• Navigability.
  
  • “The application’s user interface shall make it easy for users to navigate to where they want to go.
    Specifically, at least 90% of a statistically valid sample of users shall rate the overall user interface as being either very easy to navigate or easy to navigate on the following scale (very easy to navigate, easy to navigate, neutral, difficult to navigate, very difficult to navigate).”
  • “The application’s user interface shall make it easy for users to navigate to major functions.
    Specifically, every screen/webpage shall have a standard primary navigation area allowing users to directly go to the screens/webpages where they can perform tasks X, Y, and Z.”
• Preferance.
  
  • “The application shall be the preferred to the previous system.
    Specifically, at least 90% of a statistically valid sample of users shall rate the application as being either highly preferable or preferable to the previous application on the following scale (high preferable, preferable, neutral, not preferable, highly not preferable).”
  • “The application shall be the preferred to the competition.
    Specifically, at least 90% of a statistically valid sample of users shall rate the application as being either highly preferable or preferable to competing applications X, Y, and Z on the following scale (highly preferable, preferable, neutral, not preferable, highly not preferable).”
  • “The application shall lead its users recommend it to their friends.
    Specifically, at least 70% of a statistically valid sample of users shall state that they are willing to recommend the application to their colleagues, friends, and family.”
• Retrievability.
  
  • “The application shall enable users to retrieve data in the form they want.
    Specifically, at least 90% of a statistically valid sample of users shall state that the application allows them to obtain data in the form they want.”
  • “The application shall enable users to download music files in their favorite formats.
    Specifically, the application shall allow users to download music files in .wav, .au, and .mp3 formats.”
  • “The application shall enable users to organize the productivity matrix in report X.
    Specifically, the application shall allow users to order the columns of the productivity matrix in report X in any order they desire.”
• Suitability.
  
  • “The application shall be suitable for experienced users to use.
    Specifically, at least 90% of a statistically valid sample of users with a minimum of 1 year of experience with the previous system and 1 month of experience with the application shall state that it is either highly suitable or suitable for performing their tasks on the following scale (highly suitable, suitable, neutral, suitable, highly not suitable).”
  • “The application shall be suitable for novice users to use.
    highly suitable or suitable for performing their tasks on the following scale (highly suitable, suitable, neutral, suitable, highly not suitable).”
• Understandability.
  
  • “The application’s user interface shall be clear to its users.
    Specifically, at least 90% of a statistically valid sample of users shall rate the overall user interface as being either very clear or clear on the following scale (very clear, clear, neutral, unclear, very unclear).”
  • “The application’s user interface shall be intuitive to the users.
    Specifically, at least 90% of a statistically valid sample of users shall rate the overall user interface as being either veru intuitive or intuitive on the following scale (very intuitive, intuitive, neutral, unintuitive or counterintuitive, very unintuitive or counterintuitive).”
  • “The application’s help facilities shall be unambiguous to the users.
    Specifically, at least 90% of a statistically valid sample of users shall rate the help facilities as being either very unambiguous or unambiguous on the following scale (very unambiguous, unambiguous, neutral, ambiguous, very ambiguous).”
  • “The application’s user-oriented error messages shall be easy for its users to understand.
    Specifically, at least 90% of a statistically valid sample of trained users shall be able to correctly interpret 95% of a random selection of 20 of the application’s user-oriented error messages.
  • “The application’s user-oriented error messages shall be easy for its users to understand.
    Specifically, at least 90% of a statistically valid sample of novice users shall be able to correctly interpret 80% of a random selection of 20 of the application’s user-oriented error messages.
  • “The application’s user manual shall be logically organized.
    Specifically, at least 90% of a statistically valid sample of users shall rate the user manual as being either very logically organized or logically organized on the following scale (very logically organized, logically organized, neutral, illogically organized, very illogically organized).”
• User-Satisfaction.
  
  • “The application’s shall produce a high degree of satisfaction in its users.
    Specifically, at least 90% of a statistically valid sample of users shall rate themselves as being either very satisfied or satisfied with the it on the following scale (very satisfied, satisfied, neutral, unsatisfied, very unsatisfied).”
  • “The application shall be beneficial to its users.
    Specifically, at least 90% of a statistically valid sample of users shall rate the overall application as being either very beneficial or beneficial on the following scale (very beneficial, beneficial, neutral, not beneficial, very unbeneficial).”

Guidelines

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

• The scope of a usability requirement can be:
  • A business enterprise
  • An application
  • A component
  • A center
• 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).