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Standard deviation is a statistical measure quantifying the degree of variation in investment returns. It helps investors assess volatility and risk by showing how much returns deviate from the average. By calculating standard deviation, investors gain insight into an investment's overall risk profile, enabling them to make informed decisions aligned with their risk tolerance and investment goals.

Standard deviation is calculated by taking the square root of variance. Variance represents the average of squared deviations from the mean return. This transformation makes standard deviation more intuitive, expressing volatility in the same units as the original data, typically percentage points, making it easier for investors to interpret risk levels.

A higher standard deviation indicates greater risk and wider variations in returns. Investments with high standard deviations are more volatile, meaning returns can fluctuate significantly from the average, creating potential for both substantial gains and considerable losses. This unpredictability makes such investments riskier for investors seeking stable returns.

When comparing stocks, the stock with the lower standard deviation has more consistent, less volatile returns, indicating lower risk. Conversely, a stock with higher standard deviation experiences wider return fluctuations from the average. For example, if Stock A has a standard deviation of less than 2.5% and Stock B has 56.3%, Stock B carries significantly higher risk due to its greater volatility.

Standard deviation treats all deviations from the mean—both positive and negative—as risk. This means above-average returns, which benefit investors, are also considered risky. Consequently, standard deviation does not differentiate between upside and downside volatility, potentially leading to conservative risk assessments when positive returns are frequent.

Standard deviation enables investors to compare the risk profiles of different stocks objectively. A lower standard deviation suggests more predictable, stable returns suitable for risk-averse investors, while higher standard deviation indicates greater volatility for those with higher risk tolerance. Understanding the relationship between risk and return helps investors align their portfolio choices with personal investment objectives.

Standard deviation is expressed in the same units as the original data—typically percentage points—because it is derived by taking the square root of variance. This transformation makes the measure more intuitive and directly comparable to return data. Investors can immediately understand volatility in familiar terms, such as percentage fluctuations, rather than squared deviations.