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oGBS_IS( ) Example - Equity Put Option


Consider a European put option on a stock that pays no dividends and has a volatility of 25%. The option matures on 1 March 2003, has an exercise rate of $60.00, and a market value of $1.82. The risk-free interest rate (on an actual/365 basis) is 5.25%. What is the implied spot of the stock as at 1 March 2002?



Function Specification

=oGBS_IS(2, 1.82, "1/3/02", "1/3/03", 60, 0.25, 0.0525, 0.0525)




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


When calculating implied spot prices, the Newton-Raphson iteration procedure uses the strike rate as the initial estimate of the spot price, i.e., x0 = $60. The procedure will iterate using more and more precise estimates of the spot price until the difference between the option value derived from the spot price 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 9 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 S = X = 60, vol = 0.25, b = r = 0.0512 (see special cases), and T =1 (365/365 days), d1 = 0.3297 and d2 = 0.0797.



As iPC = -1 (put), N(d1) is 0.3708 and N(d2) is 0.4682 (see oCumNorm( ) function), the oGBS( ) equation gives the following solution:


Equation Template


Since $4.4441 is above the market value of the option, $1.82, the spot price of $60 is too low. The oGBS( ) value is therefore computed at a higher spot price, 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 spot estimate of 67.0764, the oGBS( ) equation returns $2.3891. As this value is 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 7th iteration at an implied spot price of $70.00.

Equation Template

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