For one thing, they're based on historical data, and the future could very well differ from the past. These measures may give a feeling of precision that may not be there. Moreover, experts disagree on how best to portray risk statistically. Nevertheless, these numerical measures undoubtedly capture some aspects of risk. Following are brief descriptions of some of them and the problems they were designed to solve.
Larger values indicate riskier investments.
The standard deviation tells you the range of actual returns that the investment experienced two-thirds of the time. For example, two funds may have the same average return. But the fund with the higher standard deviation had more volatile returns. For example, a fund with a beta of 1.10 would be expected to rise or fall about 10 percent more than the market. A beta of .5 means that the investment would be expected to rise or fall at about half the market's rate.