Robustness
Software is said to be robust when it performs well under unexpected conditions which stress the designer’s assumptions, as well as under normal conditions.
So says Eric Raymond in his book The Art of Unix Programming. A user interface could be considered robust when it predicts the ways in which various people will attempt to carry out their tasks.
For example, if I wanted to move this sentence to the beginning of my post, there are several methods I may attempt to do so. I would first have to select the sentence. I could accomplish this by inserting my cursor at one end of the sentence and then shift-clicking at the other end. This works in both directions. I could also click at one end of the sentence and drag to the other end. Once selected, I could choose “Cut” from the “Edit” menu, insert my cursor at the beginning of the post, and then select “Paste”. Or, I could press Command (Control on Windows) X on the keyboard and then Command V. Or I could right-click the selection and use the same commands in the context menu. Or I could click and hold on the selection for a brief second and then drag the text and drop it where I want it.
The result of a robust user interface is that users will consider it to be intuitive. They will say that it is easy to understand, because it did what they were expecting it to do.
