However finance theory assumes idealistic models for the stock markets and formulates the investor utility functions and expectations accordingly. These models are based on perfect competition and passage of information in an unfettered manner. As Wikipedia (2007) seems to point out, "In economics and financial theory, analysts use random walk techniques to model behavior of asset prices, in particular share prices on stock markets, currency exchange rates and commodity prices. This practice has its basis in the presumption that investors act rationally and without bias, and that at any moment they estimate the value of an asset based on future expectations. Under these conditions, all existing information affects the price, which changes only when new information comes out. By definition, new information appears randomly and influences the asset price randomly.
Empirical studies have demonstrated that prices do not completely follow random walk. Low serial correlations (around 0.05) exist in the short term; and slightly stronger correlations over the longer term. Their sign and the strength depend on a variety of factors, but transaction costs and bid-ask spreads generally make it impossible to earn excess returns. Researchers have found that some of the biggest prices deviations from random walk result from seasonal and temporal patterns. ...Show more