Let Z1,Z2,....Zn be independent and identically distributedrandom variable, satisfying E[ι Zt ι]<∞. Let N be an integer valued random variable whose value n depends only on the values of the first n Z¡'s. Suppose E(N)< ∞, then E(Z1,Z2,....Zn)=E(N)E(Z) is called?