Limitations of the Sharpe Ratio
Although it's a helpful tool to try and eliminate funds and fund managers that have generated outsized returns by continually taking on excess risk, the ratio could be better. It should only be used in combination with several qualitative and quantitative assessments.
For example, it's entirely possible to generate an excellent Sharpe ratio in periods when the market favors a fund's sector bias or investment style -- only to walk into severe drawdown when, say, the housing or tech sector bust occurs, or conditions are no longer suitable for trend following or mean reversion investment strategies.
Similarly, the fact that interest rates (the risk-free rate, Rf) are used to calculate the Sharpe Ratio doesn't tell you much about how the fund might perform when interest rates have significantly changed from when the ratio was calculated.
Standard deviation is also subject to significant criticism as a measure of risk.
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