What Is the Sharpe Ratio? Why the Number Alone Is Not Enough

The Sharpe ratio divides the mean return above a benchmark by the standard deviation of that return difference. When the benchmark is a risk-free return, subtract the matching-period risk-free return from each portfolio return before calculating the mean and standard deviation.

It is neither a profit amount nor a win rate. Greater variability can reduce the ratio for the same mean return. It does not guarantee capital preservation or future profits.

Do not subtract an annual interest rate directly from a daily portfolio return. Align the periods and check whether returns are simple or logarithmic and whether costs have already been deducted. Recording the standard-deviation convention also helps explain differences between implementations.

The basic form is mean excess return divided by the standard deviation of those same-period excess returns. A zero standard deviation does not produce an ordinary finite ratio. Inspect the reason for an invalid result rather than replacing it with a favorable number.

A common conversion multiplies a daily ratio by √252 under appropriate assumptions. Irregular observations or weekly returns cannot simply be treated as daily. Serial correlation can also make a simple conversion inappropriate.

For a fictional calculation example, a mean daily excess return of 0.05% and a standard deviation of 1% give a daily ratio of 0.05. Assuming 252 periods and the simple scaling convention gives about 0.79. This illustrates arithmetic, not actual KabuForge performance.

In a short sample, a handful of large moves can substantially change the mean and standard deviation. A displayed value does not imply enough evidence for comparison. Inspect observation counts, missing values and continuity.

Computing multi-session forward returns every day can create overlapping windows. Treating them as independent daily observations misrepresents the evidence. Out-of-sample evaluation also matters, but does not remove estimation uncertainty.

The Sharpe ratio summarizes the mean and variability of period returns. Maximum drawdown examines the largest percentage decline from a running peak in account value over the test period. Equal ratios can coexist with different sequences of losses and recovery times.

Read returns, the equity curve, drawdown, turnover and costs together. Check the transaction-cost article when determining whether returns already include costs.

The public course analyzes returns between adjacent account-value observations. N observations produce N−1 intervals; some summaries exclude the change from initial cash to the first observation. Their denominator need not match the original report’s full-period return.

The 252-period annualization convention applies when the report explicitly declares daily frequency. Follow the results-reading guide to inspect frequency, period, price basis, costs and calculation scope first. See William F. Sharpe’s original explanation for the definition.

KabuForge GUI research courses.

This article teaches research procedures. It does not recommend trades or guarantee future returns.Disclaimer

William F. Sharpe’s original 1994 article。 The original article discusses differential returns, variability and time dependence.

Explore the risk and return learning path

• CAGR: separate compound growth from the path of losses
• Volatility: align return frequency and standard-deviation conventions
• Sortino ratio: define the target and downside denominator