Volatility is defined as the price range for a period, divided by the average price for the period:
For instance, on 12/17/2010 the average price was 8.13 with a range of 0.21, indicating a Daily Volatility of 2.5%. Daily Volatility (the daily range as a portion of the daily average price) is plotted in green. Blue is Weekly Volatility, which is the weekly range as a portion of the weekly average price. It is important to avoid confusing this with the weekly average of the daily volatility, which is a completely different concept.
The red plot shows the actual price. The remaining marks on the plot correspond to the Volatility measured across several time intervals.
According to academic theory, Volatility is exactly equal to Risk. But investors often make a distinction between these two concepts. The different character of Volatility as seen in different time frames, lends some support to the investor's view. During 8 years, the Average Quarterly Volatility of DF stock price has been 24% while the Average Daily Volatility has been 2%.
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