Usability

Introduction
Usability [1] means to make users feel more effective, efficient and satisfied when using a product or system. According to the international standard, ISO 9241-11, usability is defined as:
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.For instance, in Internet website design, we would like to let users not to have any pressure or frustration in web surfing and allow users to use a minimum effort for gaining the maximum performance during their using the site function.
General design guidelines for usability
[2] Usability expert Jakob Nielsen and computer science professor Ben Shneiderman have separately commented about the acceptable degree of the framework of a system. It is stated that usability is part of the “helpfulness” and it contains the following elements:? Learnability: The measure of how a user can effectively use a product or system in the first time.
? Efficiency: The speed that a user learns how to practice the system and perform the task.
? Memorability: The design can be navigated and guide the user to seek what they look for, furthermore, users can recognize and remember when they return.
? Errors: How many errors do users make, how severe are these errors, and how easily can they be recovered from the errors? Satisfaction: How pleasurable is it to use the product or system
Nowadays, users always control the system by user interface, thus a good interface affects the usability of a good system. There are many good user interface design definitions on the internet and we can summarize them in seven laws[3].1. Law of clarity: Prevent the interface from having no clear meaning as the human nature would lead users to ignore the things which they do not understand.
2. Law of preferred action: Users would feel comfortable when they know what the preferred action is.
The following diagram…