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# Historical Volatility Index

**HistoricalVolatilityIndex(Vector, Periods, Bar History, Standard Deviations)HVI(Vector, Periods, Bar History, Standard Deviations)**

**Overview**

Historical volatility is the log-normal standard deviation. The Historical Volatility Index is based on the book by Don Fishback, “Odds: The Key to 90% Winners”.

The formula for a 30-day historical volatility index between 1 and 0 is: Stdev(Log(Close / Close Yesterday), 30) * Sqrt(365)

Some traders use 252 instead of 365 for the bar history that is used by the

square root function. The Log value is a natural log (i.e. Log10).

**Interpretation**

High values of HVI indicate that the stock is volatile, while low values of HVI indicate that the stock is either flat or trending steadily.

**Recommended Parameters**

Vector: CLOSE

Periods: 15

Bar History: 30

Standard Deviations: 2

Example

set BUY = **HVI(CLOSE, 15, 30, 2) < 0.01**