=+22.8. Wald's equation. Let X1, X2 ,... be independent and identically distributed with finite mean, and put S ,, = X, + . . . +X ,,. Suppose that ? is a stopping time: + has positive integers as values and [T = n] € o( X] ,..., X ,, ); see Section 7 for examples. Suppose also that E[+] < 00.