Various trading strategies appear to offer non-zero alphas when we examine real world data. If indeed these alphas are positive, it could be explained by any of the following except:

1. The market portfolio is inefficient, but the market portfolio proxy used to calculate the alphas is efficient
2. Investors are systematically ignoring positive- NPV investment opportunities
3. The positive alpha trading strategies contain risk that investors are unwilling to bear but the CAPM does not capture
4. A stock's beta with the market portfolio does not adequately measure a stock's systematic risk