Standard Deviation

Standard deviation measures how far a stock's returns typically stray from their own average. A low reading means returns cluster tightly around the mean and the stock is relatively calm; a high reading means they scatter widely and the ride is bumpy. In investing it is the most common stand-in for volatility, and therefore for risk.

Investors lean on a few rules of thumb:

• The empirical rule: for roughly normal returns, about 68% of outcomes land within one standard deviation of the average and about 95% within two. That turns a single number into a range of likely results.
• Comparing risk: a fund returning 8% with a 6% standard deviation is far steadier than one returning 8% with an 18% standard deviation, even though their averages match.
• Context matters: it only describes the past sample you feed it, and real returns have fatter tails than a perfect bell curve, so extreme moves happen more often than the model implies.
• Pair with return: a high standard deviation is not automatically bad if the reward is large enough, which is exactly what the Sharpe ratio measures.

Imagine two stocks that both averaged a 10% annual return. Stock A had a standard deviation of 5%, so most years landed between roughly 5% and 15%. Stock B had a standard deviation of 20%, so its plausible range ran from about -10% to +30%. Same average, very different risk, and the bell curve for Stock B is far wider and flatter.