The Treynor Ratio, developed by Jack Treynor (1965), measures portfolio performance per unit of market (systematic) risk:
Treynor = (R − R_f) / β
where β is the portfolio's beta relative to the market index. Unlike the Sharpe Ratio, which penalizes all volatility including idiosyncratic, the Treynor Ratio penalizes only market risk — the systematic component that cannot be diversified away.
The Treynor Ratio is most appropriate for evaluating a portfolio that is one component of a broader, well-diversified portfolio — because in that context, idiosyncratic risk is already diversified away at the total portfolio level, and only systematic risk matters for the marginal contribution of each component.
Limitations
- Beta is estimated with error, especially for portfolios with unstable or time-varying market exposure.
- Beta does not capture non-linear exposures to the market (e.g., option-like payoffs from strategies with significant convexity).
- For market-neutral strategies, beta ≈ 0 makes the Treynor Ratio undefined or very large regardless of true performance quality.