Volatility

Volatility describes how much and how quickly a stock's price swings around its trend. High volatility means large, rapid moves in both directions; low volatility means a smoother, steadier path. It is usually measured as the standard deviation of returns, which is why traders treat volatility as the headline number for risk.

Investors use volatility in a few practical ways:

• Position sizing: the more volatile a stock, the smaller the position many investors take, so one sharp move cannot dominate the portfolio.
• Expectations: a stock with 30% annualized volatility can realistically swing far more in a year than one at 10%, even if both have the same expected return.
• Regimes: volatility clusters. Calm and turbulent periods tend to persist, so a sudden spike often signals a change in market mood, not a one-off.
• Not direction: volatility measures the size of moves, not which way they go. A quiet stock can still grind lower and a wild one can still trend up.

Two funds each target an 8% yearly return. The first has 10% volatility, so most years land roughly between -2% and +18%. The second has 25% volatility, so its plausible range runs from about -17% to +33%. Same target, a dramatically different ride.