Using Mathematics to Read Hands

When you can't actually put a person on a hand but have reduced his possible hands to a limited number, you try to use mathematics to determine the chances of his having certain hands rather than others. Then you decide what kind of hand you must have to continue playing. Using mathematics is particularly important in draw poker, where your main clue to what an opponent might have is what you know about his opening, calling, and raising requirements.
If, for example, you know an opponent will raise you with three 2s or better before the draw, you can resort to mathematics to determine what hand is favored to have him beat. It works out to something like three queens. Obviously, then, if you have three 3s, it's not worth calling that opponent's raise on the chance that he has specifically three 2s. But if you have something like three Ss or three 6s, the pot odds make it correct to call because now not only might you draw out on a better hand by making a full house or four-of-a-kind, but there are a few hands your opponent could have which you already have beat.