We are dependent on craft principles, folk lore, and some scientific studies. It follows that we cannot give an a priori guarantee of the performance of an interface.
Evaluation is therefore an essential part of interface development.
User interfaces will be evaluated sooner or later. If the design of the interaction is flawed then the software will not be easy to use. Thus we need to be able to evaluate the design of the interface for usability rather than evaluate the software used to implement the interface. User interface software is the same as any other software. In other words it must satisfy the same criteria as any other software element.