 There are also probabilistic methods in mathematics which are not experimental in the above sense. For example, there are properties of composite (i.e. non-prime) numbers which can be shown to hold in relation to about half of the numbers less than a given composite number. If various numbers smaller than N are selected at random and none of them bear this relation to N, then it follows that N is almost certainly prime. The level of probability here can be precisely calculated, and can be made as high as needed by picking more ‘witness’ numbers to test.