Because the Random Walk Theory stipulates it is impossible for individuals to beat the performance of the market averages in the long run it is also stipulated that the best course of action is to only invest in a portfolio that mimics the entire universe of stocks. That this is the only way to match market performance without taking on excessive risk. But this is for long-term market movements. In the short-term the Random Walk Theory may not apply. This is borne out by the fact that there are individual traders who are able to beat the market averages over a long period of time by taking advantage of short-term anomalies in asset prices.