This Product is usable – For me?

What is usable for one user might not be usable for another user. Usability is the important criteria in a product. If I use a product & I say it is usable then it might be wrong if it is a product that is built for large number of users. For example: Motorola mobile phone keypad can be usable for users with small thumb but in some countries the same phone keypad may not be usable because people might have large thumb which makes the phone keypad not usable. It is a difficult task for the developer to create an interface considering the points from customer perspective.

Points to remember while testing for usability quality criteria
- Navigation system
- Help Documentation
- Enough information wherever required
- Spelling mistakes
- Grammatical mistakes
- What can go wrong while typing or by mistake pressing some key that might impact on using the product
- What might go wrong with expert user
- Is there anything that might have different meanings in different contexts?
- What makes our product robust? (A customer not finding obvious bug)
Note that as a tester you should not neglect any of the quality criteria.