Ok, on a typical school test, we have four students. It is a test of only two questions, A and B. Here's what happened:
When students are given "scores" on tests, that almost always means the number of questions or exercises they got "correct". So, here X has 0 and W has 2, while Z and Y each have 1. So, Z and Y are the same, right? Look at the diagram: Z and Y are as different as they can be. They are just as far apart as the complete 0 and the complete 2. Worse, Z and Y are diametrically opposite each other: what one knows, the other doesn't.
