Sharpe Ratio
A number measuring the rewardtorisk efficiency of an investment,
used to create riskefficient portfolios.
The definition of the Sharpe Ratio is:
 S(x) = (r_{x}  R_{f}) / StdDev(x)
where
 x is some investment
 r_{x} is the average annual rate of return of x
 R_{f} is the best available rate of return of a "riskless" security (ie cash)
 StdDev(x) is the standard deviation of r_{x}
See the main article on the Sharpe Ratio for more information.
