Usability Testing: How to Measure Usability Score
Introduction to Usability Testing
Usability testing is a critical aspect of user experience (UX) design that involves evaluating a product by testing it on real users. The goal is to uncover usability issues, gather qualitative and quantitative data, and improve the overall user experience. There are two main types of usability testing: moderated and unmoderated.
- Moderated Usability Testing: A facilitator guides the participant through tasks, either in person or remotely. This approach allows for real-time interaction and follow-up questions, providing deep insights into user behavior and pain points. However, it is resource-intensive and can introduce moderator bias.
- Unmoderated Usability Testing: Participants complete tasks independently, often remotely. This method is scalable and less costly but may lack the depth of insights provided by moderated sessions and can be prone to misinterpretation of user actions without the ability to ask follow-up questions.
Key Methods and Approaches
- Heuristic Evaluation: A usability inspection method where experts use established heuristics to identify usability problems in the user interface design. It is cost-effective and can be performed early in the design process. However, the quality of the results depends heavily on the evaluators’ expertise.
- User Testing: Direct interaction with users who perform specific tasks on the product. This can be done in various settings, such as in a lab, in the field, or remotely. It provides valuable insights into how real users interact with the product and where they encounter difficulties.
Measuring Usability with the System Usability Scale (SUS)
The System Usability Scale (SUS) is a reliable tool for measuring usability. It consists of a 10-item questionnaire with five response options ranging from “Strongly agree” to “Strongly disagree.” SUS provides a single score representing a composite measure of the overall usability of the system being tested.
Key Takeaways:
- Scoring: The SUS score ranges from 0 to 100, with higher scores indicating better usability.
- Thresholds: A score of 75 is generally considered “Acceptable,” suggesting that users find the system usable. Scores below 60 may fall into the “Marginally Acceptable” category, implying room for enhancement.
- Calculation: To calculate the SUS score, subtract 1 from odd-numbered question responses and subtract the response from 5 for even-numbered questions. Add up the scores for all 10 statements to obtain the individual SUS score.
- Context: The SUS score is not a diagnostic tool; it doesn’t identify specific usability issues. Instead, it provides a general indication of overall usability. To pinpoint areas for improvement, combine SUS scores with usability study results.
- Comparison: The SUS score allows for comparisons between systems, even those with different designs or functionalities. This enables organizations to evaluate and improve their usability over time.
- Normative Data: The Bangor, Kortum, and Miller study (2008) provides normative data, enabling SUS ratings to be positioned relative to other systems.
- Adjective Ratings: The SUS score can be correlated with adjective ratings, such as “Excellent” (80+), “Good” (60–79), and “Fair” (40–59).
Practical Applications of SUS
The SUS score is versatile and can be used in various contexts:
- Evaluate the effectiveness of usability improvements: By comparing SUS scores before and after changes, organizations can assess the impact of their design enhancements.
- Inform business decisions about UX investments: SUS scores provide quantitative data that can justify further investments in UX design.
- Compare usability across different systems or products: Organizations can benchmark their products against competitors or previous versions using SUS scores.
- Identify areas for improvement: Although the SUS score itself doesn’t diagnose specific issues, it highlights the need for deeper usability analysis, guiding where further testing should focus.
Best Practices for Usability Testing
- Define Clear Objectives: Establish what you aim to learn from the usability test. Whether it’s identifying pain points in the user journey, validating a design, or understanding user preferences, having clear goals will guide the testing process.
- Select Representative Participants: Ensure your test participants reflect the end users of your product. This diversity helps uncover a wide range of usability issues.
- Create Realistic Scenarios: Develop tasks and scenarios that mirror real-world use cases. This approach helps in understanding how users naturally interact with your product.
- Analyze and Prioritize Findings: Categorize issues based on their severity and impact on user experience. Focus on critical issues that hinder user tasks and address them first.
- Iterate and Test Again: Usability testing is an iterative process. Continuously test, analyze feedback, and improve your product to enhance its usability.
Conclusion
Usability testing is a crucial aspect of UX design, ensuring that products are intuitive and fulfill user requirements. Techniques like heuristic evaluation and tools such as the System Usability Scale (SUS) enable designers to gather valuable insights and make data-driven refinements to their products. By following best practices and emphasizing user feedback, teams can enhance the usability and overall user experience of their designs.
