Beta

Beta is a number that describes how much a stock tends to move compared to the overall market, usually represented by an index like the S&P 500. It answers a simple question: when the market moves 1%, how much does this stock typically move?

Beta is calculated by comparing a stock's historical price changes to the market's price changes over some period, using a statistical method (regression) that fits a line through the two sets of returns. The slope of that line is the beta. A beta of 1 means the stock has, on average, moved in step with the market. A beta of 1.5 means it has tended to move about 50% more than the market in either direction; a beta of 0.5 means moves that were roughly half the market's size. Negative beta, though uncommon, means the stock has tended to move opposite the market.

The nuance that trips people up is that beta is backward-looking and statistical, not a promise. It's built from past price data over a chosen window (often two to five years of monthly returns, but the exact calculation varies by data provider), and different providers can report different betas for the same stock because they used different time periods or benchmarks. Beta also says nothing about a stock's own company-specific risk — a stock can have a low beta and still be volatile for reasons that have nothing to do with the market, such as an earnings surprise or a lawsuit. It measures sensitivity to broad market swings, not overall riskiness.

Beta is also not the same as correlation. A stock could have a high beta but a weak correlation to the market, meaning that on the days it does move with the market it moves a lot, but on many days it moves for reasons unrelated to the market at all.