We have seen in Section 3.4 that if the joint d.f. F of two random variable X and Y is known, then the d.f. F of the random variable X can be derived from F. In this context, where the distribution of X is derived from the joint distribution of X and Y, F is called the marginal d.f. of X. Similarly, if the joint p.f. , joint p.d.f. , or joint p.f./p.d.f. f of X and Y is known, then the marginal p.f. or marginal p.d.f. of each random variable can be derived from f.