  Let X_1, X_2,..., X_n be a sequence of independent random variables, let M be
a rearrangement invariant space on the underlying probability space, and let N
be a symmetric sequence space. This paper gives an approximate formula for the
quantity || ||(X_i)||_N ||_M whenever L_q embeds into M for some 1 le q <
infty. This extends work of Johnson and Schechtman who tackled the case when N
= l_p, and recent work of Gordon, Litvak, Schuett and Werner who obtained
similar results for Orlicz spaces.
