Historical volatility (HV) is a statistical measure of the dispersion of returns for a given security or market index over a given period of time. Generally, this measure is calculated by determining the average deviation from the average price of a financial instrument in the given time period. Using standard deviation is the most common, but not the only, way to calculate historical volatility. The higher the historical volatility value, the riskier the security. However, that is not necessarily a bad result as risk works both ways—bullish and bearish.