sentences:  As a result, Malkiel argued, stock prices are best described by a statistical process called a "random walk" meaning each day's deviations from the central value are random and unpredictable.This led Malkiel to conclude that paying financial services persons to predict the market actually hurt, rather than helped, net portfolio return.A number of empirical tests support the notion that the theory applies generally, as most portfolios managed by professional stock predictors do not outperform the market average return after accounting for the managers' fees.