William Sharpe — Creator of CAPM and the Sharpe Ratio
The Capital Asset Pricing Model (CAPM)
Published in 1964, CAPM provides a formula for calculating the expected return of any asset based on its systematic risk (beta) relative to the market. It’s the foundation of how Wall Street prices risk.
| Variable | Meaning |
|---|---|
| E(Rᵢ) | Expected return of the asset |
| Rf | Risk-free rate (typically T-bills) |
| βᵢ | Beta — the asset’s sensitivity to market movements |
| E(Rm) − Rf | Market risk premium — the extra return investors demand for holding the market instead of risk-free assets |
What CAPM Tells Us
- Only systematic risk (market risk, measured by beta) is compensated with higher returns
- Unsystematic risk (company-specific risk) can be eliminated through diversification — so investors aren’t paid for bearing it
- Higher beta = higher expected return, but also higher risk
- An asset with beta of 1.0 should return the market rate; beta of 1.5 should return 50% more excess return
The Sharpe Ratio
The Sharpe Ratio measures risk-adjusted performance — how much excess return you get per unit of total risk. It’s the most widely used metric for comparing investment performance.
| Sharpe Ratio | Interpretation |
|---|---|
| Below 0 | Portfolio underperforms the risk-free rate — bad |
| 0 – 1.0 | Sub-par risk-adjusted returns |
| 1.0 – 2.0 | Good risk-adjusted returns |
| 2.0 – 3.0 | Very good — rare for long periods |
| Above 3.0 | Exceptional — verify the data, it may be too good to be true |
Impact on Modern Finance
| Application | How Sharpe’s Work Applies |
|---|---|
| WACC Calculation | CAPM is the standard method for estimating the cost of equity in corporate finance |
| Fund Evaluation | The Sharpe Ratio is the go-to metric for comparing mutual funds and hedge funds |
| Beta Analysis | Every stock screener shows beta — a direct product of CAPM |
| Alpha Measurement | Alpha (excess return beyond CAPM prediction) is how active managers prove their worth |
| Security Market Line | Plots expected return vs. beta — assets above the line are “cheap,” below are “expensive” |
Criticisms of CAPM
- Single factor: Only uses market beta — ignores size, value, momentum, and other factors identified by Fama-French
- Assumes perfect markets: No transaction costs, taxes, or information asymmetry
- Beta instability: A stock’s beta changes over time, making forward estimates unreliable
- Market portfolio: The “true” market portfolio (all investable assets) is unobservable in practice
Key Takeaways
- William Sharpe developed CAPM, which links expected return to systematic risk (beta)
- The Sharpe Ratio measures excess return per unit of risk — the standard for comparing investments
- CAPM is used daily to calculate cost of equity, evaluate alpha, and assess portfolio performance
- His work extended Markowitz’s portfolio theory into a practical pricing framework
- Sharpe shared the 1990 Nobel Prize with Markowitz and Merton Miller
Frequently Asked Questions
What is the Capital Asset Pricing Model?
CAPM calculates the expected return of an asset based on its beta (sensitivity to market risk), the risk-free rate, and the market risk premium. It’s the foundation of modern asset pricing.
What is a good Sharpe Ratio?
A Sharpe Ratio above 1.0 is generally considered good, above 2.0 is very good, and above 3.0 is exceptional. Most diversified portfolios over long periods achieve Sharpe Ratios between 0.5 and 1.5.
What is beta in CAPM?
Beta measures how much an asset’s returns move relative to the market. A beta of 1.0 means the asset moves with the market; above 1.0 means it’s more volatile; below 1.0 means less volatile.
How is CAPM used in corporate finance?
CAPM is the standard method for estimating a company’s cost of equity, which feeds into the weighted average cost of capital (WACC) — used in DCF models and investment decisions.
What’s the difference between the Sharpe Ratio and Sortino Ratio?
The Sharpe Ratio uses total standard deviation (both upside and downside), while the Sortino Ratio only penalizes downside deviation. Sortino is arguably better because investors don’t mind upside volatility.