Fama and French Three Factor Model
An asset pricing model (actually a modification of CAPM) designed by Gene Fama and Ken French. This model considers the fact that two particular types of stocks outperform markets on a regular basis: value and small-caps.
Investopedia Commentary
In simplified terms, this model punishes stocks classified as small-cap or value (defined as a stock with a high book value to market value). Fama and French are strong supporters of the Efficient Market Hypothesis. They believe that you only get excess return for taking on extra risk. Thus, if small-caps or value stocks have a higher than average return, then they must be riskier.
Critics of this theory see the Three Factor Model as academic nonsense (to be blunt). The biggest problem is that Fama and French don't really say why the size of a company or it's book-to-market ratio is a proper indicator of risk. In fact, for a value investor, a high book-to-market ratio means there is less risk because the stock is "cheap" and perhaps undervalued by the market.
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See also: CAPM, Efficient Market Hypothesis, Small-Cap, Value Stock