之前看過 User-Centered Web Design (John Cato) 的書，本書作者提及到 Usability 的定義 Usability is being able to do the things you want to, not the things you have to! 簡單來講就是當設計用戶介面時不要硬板板，要靈活，使用者在使用時唔覺得是一項工作．都是有點抽象，所以本書作者提出以下八個方向去量度 Usability 使其具體化。
1. Learnability (容易學習)
The system should be easy to learn, so the user can quickly get some work done.
2. efficiency (高效率)
Once users have learned the system, they should be able to use it productively.
3. memorability (容易記下來)
The system should be easy to remember, so the casual user is able to use the system again without having to relearn everything.
4. errors (知道自己的錯誤)
The system should have a low error rate, so the user feels they are making positive progress and are in control, and if they do make errors they should be able to recover from them easily. Catastrophic errorss should not occur.
5. satisfaction (覺得稱心舒服)
The system should be pleasant to use, so users are subjectively satisfied when using it.
6. control (要控制多於被控制)
Users feel they are in control rather than the system controlling them.
7. skills (提高用者技能)
Users feel that the system support, supplements and enhances their skills and expertise – it has respect for the user.
8. privacy (保護用者私隱)
The system helps users to protect information belonging to them or their clients.