While designing an interface, the key factor is understanding the expected user. Is your application a general-purpose program that could be used by anyone in the population? Or, is it targeted for a specific category of users, such as doctors, who will be expected to know certain things about the program?
The user case can help determine how to best layout a GUI, what information to include on different screens, and even what type of interaction device will be used, such as a mouse pointer versus a fingertip. In the case of the doctor example, we may decide that the doctors will use the GUI on a tablet, so their fingers will be the main way to interact with the interface. Thus, buttons should be larger and spread apart more ...
