What is beta (β) in stock investing?
Beta (β) measures how sensitive a stock's returns are to S&P 500 returns, estimated via OLS regression over a rolling 1-year window of daily returns. β = 1.0 means the stock tends to move in line with the index. β > 1 amplifies both gains and losses — a β of 1.5 tends to move about 50% more than the index in either direction. β < 1 dampens moves relative to the index; a negative β would mean the stock tends to move opposite the market, which is rare among S&P 500 constituents.
What beta does and doesn't capture
Beta describes historical co-movement with the market, not a company's fundamentals or its own idiosyncratic risk. Two stocks can share the same beta while having very different volatility profiles driven by stock-specific events (earnings, litigation, management changes) that don't correlate with the broad market at all. Beta is also unstable over time — a name can re-rate to a materially different beta after a business-model shift, which is why Quantustik recomputes it on a rolling window rather than treating it as a fixed constant.
Live example: AAPL's current beta is 1.10 — close to market-average volatility. See the full AAPL forecast to see how this feeds its position-sizing recommendation.
How beta feeds the quantum model and position sizing
Beta is an input to the quantum model's volatility calibration and to the Kelly position-sizing calculation: a higher-beta name generally warrants a smaller position for the same dollar-risk budget, all else equal, because its price swings (and therefore its stop-loss distance) tend to be larger.
Frequently asked questions
What does a beta of 1.5 mean?
A beta of 1.5 means the stock has historically moved about 50% more than the S&P 500 in either direction — amplifying both gains and losses relative to the index.
Can beta be negative?
Yes in theory — a negative beta means a stock tends to move opposite the broad market — but this is rare among S&P 500 constituents, most of which have positive beta.
Does a low beta mean a stock is safe?
Not necessarily. Low beta means low correlation with market-wide moves, but a stock can still carry large idiosyncratic risk from company-specific events that beta alone doesn't capture.
Educational research only — not investment advice.