Proponents of market efficiency divide risk into unsystematic and systematic. Unsystematic risk is not priced by everyone investing in the stock market. Here is an example to help you understand unsystematic risk. If you are considering investing in the stock market you could either buy specific stock in a specific company that you think will have a rise in price in the future. On the other hand if you dont trust your stock ability you have the alternative of buying a basket of stocks that mimics the stock markets total combined movement. One way would to be to buy an indexed mutual fund like VFINX which is pegged to the S&P 500 which is a very large stock market index. The degree to which the stock moves relative to the general market is the unsystematic risk of the stock.

Systematic risk is the degree to which the stock changes in price relative to the general stock market as measured by an index like the S&P 500. Model calls this measure a stocks beta. The Fama-French Three Factor Model is a regression analysis that tries to separate out the systematic risk of a stock from the unsystematic risk by compensating for three factors. The first factor is a financial ratio called book to market. The second factor is the size of the firm as measured by its market capitalization. The third factor is the return on the market portfolio.

The book to market ratio is nothing more than what accountants estimate the company to by worth divided by the market capitalization of the company. The market capitalization of the company is the share price of the stock times the total number of shares the company has outstanding in the stock market. The return on the market portfolio is measured by some index like the S&P 500.

According to the efficient market school (which I do not agree with), size and book to market reflect systematic risk, meaning risk that requires compensation in the form of higher expected returns. If this is the case researchers should see that investors perceive small-value stocks to be riskier than large-growth stocks. The do see this which does lend some support to market efficiency. But investors consistently expect large-value stocks to outperform small-growth stocks and this is perverse. Basically, investors recognize that small upcoming companies are riskier but do not expect to be compensated for this risk as the efficient market model says that they should.

In a similar fashion, analysts tend to recommend growth stocks more favorably than they do value stocks. In the efficient market model of which the capital asset model (CAPM) is a part of, the profit from stock investing that investors expect should be as much as the risk they perceive that they are taking instead of the exact opposite which we find to be the case when actual research is performed on the matter.

This result caused the death of CAPM beta that was treasured by efficient market theorists despite the fact that the model resulted in the awarding of a Nobel Prize in economics to William Sharpe of Stanford University. Hirsh Shefrin has suggested that a behavioral beta be introduced into the model that might help explain these results that are contrary to market efficiency.