Usability is “the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use” (ISO 9241-11 standard).
Causes
Not specified in notes.
Goals / Objectives
Provide a standardized framework to evaluate how well users can interact with a system.
Importance
Forms the foundation for all usability evaluation and measurement efforts.
Guides design and assessment across diverse contexts and user groups.
Benefits
Offers a clear, three-dimensional structure (effectiveness, efficiency, satisfaction) for evaluating user experience.
Procedures
Not specified in notes as a process; presented as a definition.
Advantages & Disadvantages
Not specified in notes.
Impact / Effect
Shapes how usability is understood, measured, and improved in both academic and industry settings.
Examples
Not provided as a specific example, but the three components are elaborated in subsequent subtopics.
Effectiveness
Definition
The accuracy and completeness with which users achieve specified goals.
Causes
Not specified in notes.
Goals / Objectives
Measure whether users can correctly and fully complete tasks.
Importance
If users cannot accomplish their goals, other aspects of usability become irrelevant.
Benefits
Provides clear diagnostic data about task success and system reliability.
Procedures
Define success criteria for each task before data collection.
Record binary outcomes (1 = complete, 0 = failed).
For accuracy: count errors (e.g., spelling mistakes).
For completeness: calculate ratio of completed work to total required work (e.g., words transcribed ÷ total words).
Advantages & Disadvantages
Not explicitly contrasted in notes.
Impact / Effect
Directly influences user trust and system adoption.
Errors often map to specific UI problems needing redesign.
Examples
Transcribing a 2-page document into PDF:
Accuracy: number of spelling mistakes made.
Completeness: (number of words transcribed) ÷ (total words in source).
Task: “Find the current price for a share of Google stock” (clear success criterion).
Efficiency
Definition
The resources (time, effort, actions) expended in relation to the accuracy and completeness with which users achieve goals.
Causes
Not specified in notes.
Goals / Objectives
Quantify how quickly and with how much effort users complete tasks.
Importance
Reflects productivity and cognitive/physical workload during interaction.
Benefits
Enables comparison between designs, user groups, or tasks.
Supports optimization of workflows and interface elements.
Procedures
Measure task time: from when the user finishes reading the task scenario to when all actions are complete.
Count number of actions/steps taken per task (more steps = more effort).
Compare efficiency across:
Different product versions
Different user types
Different tasks on the same system
Advantages & Disadvantages
Not explicitly contrasted in notes.
Impact / Effect
Poor efficiency can lead to user frustration, abandonment, or errors.
High efficiency supports better user retention and performance.
Examples
Time taken on first attempt to complete a registration form.
Number of clicks required to find a product in an e-commerce app.
Satisfaction
Definition
The comfort and acceptability of use, reflecting users’ subjective experience with a system.
Causes
Not specified in notes.
Goals / Objectives
Capture users’ emotional and attitudinal responses to a system.
Importance
Determines long-term adoption, loyalty, and perceived quality.
Benefits
Reveals how users feel about control, helpfulness, and ease of learning.
Complements objective metrics (effectiveness, efficiency) with human-centered insights.
Procedures
Use standardized questionnaires:
Net Promoter Score (NPS): % of users who would recommend the product.
Customer Satisfaction Score (CSAT): % rating the product as “satisfying” or giving high ratings (e.g., stars, thumbs-up).