Luca: don't ask me to elaborate on these because all I did was find the info - lol ... from an old Motley Fool link ...
Calculating Sharpe Ratios
by Robert Sheard
(TMF Sheard)
LEXINGTON, KY. (August 20, 1998) -- One measure investors like to look at is the risk-adjusted performance of different strategies. And of the many ways of looking at risk, perhaps the most popular is the Sharpe Ratio.
The Sharpe Ratio takes into account three elements: the average return of a strategy over a given period, the rate of risk-free return one could have received during that period (Treasury bills, for example), and the standard deviation of the strategy's returns (how wild the ride can be).
The formula for calculating the Sharpe Ratio is to subtract the risk-free rate from the average return of the strategy and then divide that result by the standard deviation.
For example, let's look at the returns for the Keystone 5 from 1986 through the end of last year:
1986 22.0
1987 8.1
1988 8.5
1989 59.2
1990 (0.3)
1991 71.8
1992 12.4
1993 36.3
1994 9.0
1995 43.8
1996 38.2
1997 56.6
The average return (mean) for that period was 30.5%. The average T-bill return for those 12 years was 5.5%. Subtract that from the average strategy return, leaving 25%. Now divide that by the standard deviation of those returns (22.9%), and you get a Sharpe Ratio of 1.09.
Now for that to be meaningful, it needs to be compared with an alternative strategy or benchmark. If we use the Standard & Poor's 500 Index over the same period, the average return was 17.7%, less the risk-free return of 5.5%, leaving 12.2%. Divide that by the standard deviation for the S&P 500 Index of 13.0% and the Sharpe Ratio comes out to 0.94.
By contrast, the High Yield 10 looks this way:
Average Return for the 12 years: 19.6%
Risk-Free Return: 5.5%
Standard Deviation: 14.7%
Sharpe Ratio: 0.96
When comparing Sharpe ratios, the higher the ratio the better the risk-adjusted return, but be careful in associating this with a measure of pure risk. It's not. The Sharpe ratio measures how well an investor would have been rewarded for taking on that level of risk associated with the strategy over that particular period. It's possible to have a strategy with a high Sharpe Ratio that is still a "risky" strategy. Nevertheless, it's one good measure of how a strategy performs when compared to other approaches.