Usability
The degree to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction.
Also known as: Ease of use, User-friendliness
Category: Principles
Tags: user-experience, quality, user-centered, interfaces, evaluation
Explanation
Usability is a quality attribute that assesses how easy user interfaces are to use. It's defined by five quality components identified by Jakob Nielsen:
**The Five Components of Usability**:
1. **Learnability**: How easy is it for users to accomplish basic tasks the first time they encounter the design?
2. **Efficiency**: Once users have learned the design, how quickly can they perform tasks?
3. **Memorability**: When users return after a period of not using it, how easily can they reestablish proficiency?
4. **Errors**: How many errors do users make, how severe are they, and how easily can users recover?
5. **Satisfaction**: How pleasant is it to use the design?
**Usability vs. Utility**:
- **Utility**: Does it provide the features you need?
- **Usability**: How easy and pleasant are those features to use?
- **Usefulness**: Utility + Usability combined
A product can have high utility (many features) but low usability (hard to use), or vice versa.
**Measuring Usability**:
- **Task Success Rate**: Percentage of correctly completed tasks
- **Time on Task**: How long tasks take to complete
- **Error Rate**: Number and severity of errors
- **System Usability Scale (SUS)**: Standardized questionnaire
- **Net Promoter Score (NPS)**: Likelihood to recommend
**Usability Testing Methods**:
- **Moderated Testing**: Researcher guides participants through tasks
- **Unmoderated Testing**: Users complete tasks independently (remote)
- **A/B Testing**: Compare two versions with different users
- **Heuristic Evaluation**: Experts evaluate against principles
- **Cognitive Walkthrough**: Step through tasks from user perspective
**Why Usability Matters**:
- Users leave sites that are hard to use
- Poor usability increases support costs
- Good usability reduces training time
- Usability affects brand perception
- Accessibility and usability overlap significantly
Usability should be considered throughout the design process, not just at the end. Testing early with rough prototypes catches problems when they're cheap to fix.
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