 It is closely related to the use of independent and identically distributed random variables in statistical models.Exchangeable sequences of random variables arise in cases of simple random sampling.Formally, an exchangeable sequence of random variables is a finite or infinite sequence "X"1, "X"2, "X"3, ... of random variables such that for any finite permutation σ of the indices 1, 2, 3, ..., (the permutation acts on only finitely many indices, with the rest fixed), the joint probability distribution of the permuted sequence

is the same as the joint probability distribution of the original sequence.