By evaluating each item in the collection in such a way that any two items can then be compare to see which should come higher in the ranking. In mathematical terms, this is known as a weak order or total preorderhttp://en.wikipedia.org../../../../articles/s/t/r/Strict_weak_ordering.html#Total_preorders of objects. It is not necessarily a total order of objects because two different objects can have the same ranking. The rankings themselves are totally ordered. For example, materials are totally preordered by hardness, while degrees of hardness are totally ordered.