Make sure you answer all the questions , include everything requested and justify all the answers. An HCI professional would be required to know what the trade-offs are between design decisions.
Usability studies do help.
Many software companies these days conduct extensive usability studies on their products. As computer scientists and software engineers we need to be aware of what makes an interface difficult for the user and learn how to design and carry out usability studies. Furthermore, usability studies will help us in understanding the trade-offs between design decisions. Usability tests require hard work and good knowledge of what measurements need to be performed. For example, recently in class we looked at Fitt's law with which we can estimate the time for a user's hand to move to a destination area, such as the movement to drag a file icon to the trash bin or point the mouse to a menu item.