Add training workflow, datasets, and runbook
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Chapter 28: Mathematical Applications 463
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This is then the proper way to calculate historical volatility. Obviously, the
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strategist can calculate 10-, 20-, and 50-day and annual volatilities if he wishes - or
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any other number for that matter. In certain cases, one can discern valuable infor
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mation about a stock or future and its options by seeing how the various volatilities
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compare with one another.
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There is, in fact, a way in which the strategist can let the market compute the
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volatility for him. This is called using the implied volatility; that is, the volatility that
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the market itself is implying. This concept makes the assumption that, for options
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with striking prices close to the current stock price and for options with relatively
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large trading volume, the market is fairly priced. This is something like an efficient
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market hypothesis. If there is enough trading interest in an option that is close to the
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money, that option will generally be fairly priced. Once this assumption has been
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made, a corollary arises: If the actual price of an option is the fair price, it can be fixed
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in the Black-Scholes equation while letting volatility be the unknown variable. The
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volatility can be determined by iteration. In fact, this process of iterating to compute
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the volatility can be done for each option on a particular underlying stock This might
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result in several different volatilities for the stock If one weights these various results
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by volume of trading and by distance in- or out-of-the-money, a single volatility can
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be derived for the underlying stock This volatility is based on the closing price of all
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the options on the underlying stock for that given day.
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Example: XYZ is at 33 and the closing prices are given in Table 28-1. Each option
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has a different implied volatility, as computed by determining what volatility in the
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Black-Scholes model would result in the closing price for each option: That is, if .34
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were used as the volatility, the model would give 4¼ as the price of the January 30
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call. In order to rationally combine these volatilities, weighting factors must be
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applied before a volatility for XYZ stock itself can be arrived at.
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The weighting factors for volume are easy to compute. The factor for each
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option is merely that option's daily volume divided by the total option volume on all
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XYZ options (Table 28-2). The weighting functions for distance from the striking
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price should probably not be linear. For example, if one option is 2 points out-of-the
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money and another is 4 points out-of-the-money, the former option should not nec
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essarily get twice as much weight as the latter. Once an option is too far in- or out-of
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the-money, it should not be given much or any weight at all, regardless of its trading
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volume. Any parabolic function of the following form should suffice:
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{
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(x - a)2 if xis less than a
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Weighting factor = -;;,r-
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= 0 if x is greater than a
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