Add training workflow, datasets, and runbook
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Chapter 38: The Distribution of Stock Prices 799
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thing that cannot be predicted with certainty. Nevertheless, any probability calcula
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tor requires this input. So, one must understand that the results one obtains from any
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of these probability calculators is an estimate of what might happen. It should not be
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relied on as "gospel."
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Additionally, probability calculators make a second assumption: that the volatil
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ity one inputs will remain constant over the entire length of the study. We know this
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is incorrect, for volatility can change daily. However, there really isn't a good way of
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estimating how volatility might change in the course of the study, so we are pretty
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much forced to live with this incorrect assumption as well.
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There is no certain way to mitigate these volatility "problems" as far as the prob
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ability calculator is concerned, but one helpful technique is to bias the volatility pro
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jection against your objectives. That is, be overly conservative in your volatility pro
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jections. If things tum out to be better than you estimated, fine. However, at least
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you won't be overstating things initially. An example may help to demonstrate this
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technique.
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Example: Suppose that a trader is considering buying a straddle on XYZ. The five
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month straddle is selling for a price of 8, with the stock currently trading near 40. A
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probability calculator will help him to determine the chances that XYZ can rise to 48
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or fall to 32 (the break-even points) prior to the options' expiration. However, the
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probability calculator's answer will depend heavily on the volatility estimate that the
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trader plugs into the probability calculator. Suppose that the following information is
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know about the historical volatility of XYZ:
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l 0-day historical volatility:
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20-day historical volatility:
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50-day historical volatility:
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l 00-day historical volatility
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22%
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20%
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28%
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33%
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Which volatility should the trader use? Should he choose the 100-day historical
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volatility since this is a five-month straddle, which encompasses just about 100 trad
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ing days until expiration? Should he use the 20-day historical volatility, since that is
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the "generally accepted" measure that most traders refer to? Should he calculate a
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historical volatility based exactly on the number of days until expiration and use that?
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To be most conservative, none of those answers is right, at least not for the right
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reasons. Since one is buying options in this strategy, he should use the lowest of the
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above historical volatility measures as his volatility estimate. By doing so, he is taking
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a conservative approach. If the straddle buy looks good under this conservative
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assumption, then he can feel fairly certain that he has not overstated the possibilities
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