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How beta is calculated
Beta is the slope of the regression line between the security's returns and the benchmark's returns over a chosen period. Most published betas use trailing 60-month monthly returns or 36-month for shorter histories. Different periods produce different betas — published numbers from different sources often disagree.
What beta tells you
- How much of the security's variance is explained by the market.
- Expected sensitivity to broad market moves — useful for stress testing.
- The component of return that is "rented" from the market vs. specific to the security.
What beta doesn't tell you
- Tail behavior — beta is calculated from average sensitivity; portfolios can have low beta in calm periods and high beta in crashes (correlation rises in stress).
- Idiosyncratic risk — security-specific risk not captured by the market relationship.
- Style and factor exposures — two portfolios with the same beta can have very different value/momentum/size tilts.
- Drawdown — beta says nothing about peak-to-trough loss potential.
Beta and benchmark selection
Beta is meaningful only relative to a benchmark. A small-cap fund's beta against the S&P 500 will differ from its beta against the Russell 2000 — and the right benchmark depends on what the manager actually does. Mismatched benchmarks produce misleading beta numbers that can flatter or punish a strategy.
How StratiFi thinks about beta
Beta is a useful summary number, not the whole story. The firms that use it well pair beta with concrete drawdown analysis, stress-test scenarios, and factor exposure — the combination tells the client what the portfolio does in different environments, not just on average.
Frequently asked questions
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Can a stock have a beta below zero?
Yes, mathematically — negative beta means the security moves opposite the benchmark on average. In practice, very few individual securities have meaningfully negative betas; some commodities and inverse funds do. -
Is high beta the same as high risk?
Not quite. Beta measures sensitivity to the benchmark; risk has many other dimensions (idiosyncratic, liquidity, credit, drawdown). High-beta portfolios are typically more risky, but the mapping is not one-to-one. -
Why do different sources show different betas?
Different periods, different benchmarks, different return frequencies (daily vs. monthly), and different statistical methods all produce different betas. The number is a function of the methodology, not just the security.