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oBS_IV( ) Example 1 - Equity Call Option

Description

Consider a European call option on a stock that has a current spot price of $85. The call option has a strike price of $85, matures on 1 March 2003 and has a market value of $15.75. The risk-free interest rate (on an actual/365 basis) is 6.0%. What is the implied volatility of this option as at 1 March 2002?

 

 

Function Specification

=oBS_IV(1, 15.75, "1/3/02", "1/3/03", 85, 85, 0.06)

 

 

Solution

As there is no closed form solution for implied volatility, the Newton-Raphson iteration procedure is used to solve for vol.

 

 

When calculating implied volatilities, the Newton-Raphson iteration procedure uses the Manaster and Koehler seed value as the initial estimate of the volatility. This is calculated as follows (see below for r and T parameter values):

Equation Template

The procedure will iterate using more and more precise estimates of volatility until the difference between the option value derived from the volatility estimate and the given market option value is less than the desired accuracy level (see Newton-Raphson). In this example the desired accuracy level is 11 decimal places.

 

The continuous equivalent of the actual/365 risk-free interest rate is calculated as follows:

Equation Template

Referring to the equations for d1 and d2 (see model definition), if vol = 0.3414, S = X = 85, r = 0.0583, and T = 1 (365/365 days), d1 = 0.3414 and d2 = 0.0000.

 

 

As iPC = 1 (call), N(d1) is 0.6336 and N(d2) is 0.5000 (see oCumNorm( ) function), the oBS( ) equation gives the following solution:

 

Equation Template

 

Since $13.7608 is below the market value of the option, $15.75, the volatility of 34.14% is too low. The oBS( ) value is therefore computed at a higher volatility, i.e., x1 > x0. Referring to the Newton-Raphson iteration procedure, x1 is determined as:

Equation Template

 

Using the same parameter values as above with a new volatility estimate of 40.356%, the oBS( ) equation returns $15.7489. As this value is slightly below the market value of the option the next volatility trial is:

Equation Template

 

 

This process continues until the convergence criteria is met, which for this example occurs on the 4th iteration at a volatility of 40.3593%.

Equation Template

 

 

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