## The binomial option pricing model – part 5

This is post #5 on the binomial option pricing model. The purpose of post #5:

Post #5: Tweak the binomial European option pricing methodology to work for American options.

The work in this post is heavily relying on the work in the binomial option pricing model for European options (multiperiod, one-period and more on one-period).

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Valuing American options

The binomial tree approach of pricing options can also be used to price American options. Recall that a European option can be exercised only at expiration. An American option is one that can be exercised at any time during the life of the option. This means that in a binomial tree, an European option can be exercised only at the final nodes while an American option can be exercised at any node if it is profitable to do so. For an American option, the option value at a given node is obtained by comparing the exercise value (i.e. the value of the option if it is exercised at that node) and the intrinsic value (the value of the option resulting from the binomial model calculation). Thus for an American option, the option value at each node is simply the greater of the exercise value and the intrinsic value. The following 3-step process summarizes the approach in pricing an American option.

Pricing an American option using a multi-period binomial tree

1. Build a binomial tree.
2. Calculate the option values at the last nodes in the tree. For a call, the option value at the end of the tree is either the stock price less the strike price or $0, whichever is greater. For a put, the option value at the end of the tree is either the strike price less the stock price or$0, whichever is greater.
3. Starting from the option values at the final nodes, work backward to calculate the option value at earlier nodes. The option value at the first node is the price of the option. Keep in mind at each node, the option value is either the intrinsic value (the value calculated using the binomial pricing method) or the exercise value, whichever is the greater.

The three-step process is almost identical to the process of European option valuation discussed in binomial model post #4. The tweak is in Step 3, allowing for early exercise at any node whenever it is advantageous to do so (for the option holder).

In Step 3, we use risk-neutral pricing. The idea is that the option value at each node is the weighted average of the option values in the later two nodes and then discounted at the risk-free interest. The two option values (at the up node and at the down node) are weigted by the risk-neutral probabilities as follows:

$\displaystyle C^*=e^{-r h} \ [p^* \times (C^*)_u + (1-p^*) \times (C^*)_d] \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (1)$

where $C^*$ is the option value at a given node, and $(C^*)_u$ is the option value at the up move and $(C^*)_d$ is the option value at the down move that follow the node at $C^*$. The risk-neutral probability $p^*$ for the up move is:

$\displaystyle p^*=\frac{e^{(r-\delta) h}-d}{u-d}$

$\displaystyle d=e^{(r-\delta) h - \sigma \sqrt{h}}$

$\displaystyle u=e^{(r-\delta) h + \sigma \sqrt{h}}$

where $r$ is the annual risk-free interest rate, $h$ is the length (in years) of a period in the binomial tree, $u$ and $d$ are the stock price movement factors and $\sigma$ is the stock price volatility factor. The risk neutral pricing is discussed in binomial model post # 2.

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Examples

The binomial tree pricing process produces more accurate results when the option period is broken up into many binomial periods. Thus the binomial pricing model is best implemented in computer. In order to make a binomial tree a more realistic model for early exercise, it is critical for a binomial tree to have many periods when pricing American options. Thus the examples given here are only for illustration purpose.

Example 1
A 6-month American put option has the following characteristics:

• Initial stock price is $40. • Strike price of the put option is$45.
• The stock is non-dividend paying.
• The annual standard deviation of the stock return is $\sigma=$ 0.3.
• The annual risk-free interest rate is $r=$ 5%.

Price this put option with a 3-period binomial tree. Compare the American option with the European but otherwise identical put option.

Compare the following two binomial trees. The first one is for the American put option. The second one is for the otherwise identical European put option.

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Example 1 – the binomial tree and option values – American put
$\text{ }$
$\displaystyle \begin{array}{llll} \displaystyle \text{Initial Price} & \text{Period 1} & \text{Period 2} & \text{Period 3} \\ \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uuu}=\ 59.22258163 \\ \text{ } & \text{ } & \text{ } & C_{uuu}=\ 0 \\ \text{ } & \text{ } & S_{uu}=\ 51.96108614 & \text{ } \\ \text{ } & \text{ } & C_{uu}=\ 0 & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uud}=\ 46.3561487 \\ \text{ } & \text{ } & \text{ } & C_{uud}=\ 0 \\ \text{ } & S_u=\ 45.58994896 & \text{ } & \text{ } \\ \text{ } & C_u=\ 2.41285153 & \text{ } & \text{ } \\ S=\ 40 & \text{ } & S_{ud}=S_{du}=\ 40.67225322 & \text{ } \\ C=\ 6.024433917 & \text{ } & C_{ud}=\ 4.585624746 & \text{ } \\ \text{ } & S_d=\ 35.68528077 \text{ } & \text{ } \\ \text{ } & \mathbf{C_d=\ 9.314719233} \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{udd}=\ 36.28501939 \\ \text{ } & \text{ } & \text{ } & C_{udd}=\ 8.714980615 \\ \text{ } & \text{ } & S_{dd}=\ 31.83598158 & \text{ } \\ \text{ } & \text{ } & \mathbf{C_{dd}=\ 13.16401842} & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{ddd}=\ 28.40189853 \\ \text{ } & \text{ } & \text{ } & C_{ddd}=\ 16.59810147 \\ \end{array}$

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Example 1 – the binomial tree and option values – European put
$\text{ }$
$\displaystyle \begin{array}{llll} \displaystyle \text{Initial Price} & \text{Period 1} & \text{Period 2} & \text{Period 3} \\ \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uuu}=\ 59.22258163 \\ \text{ } & \text{ } & \text{ } & C_{uuu}=\ 0 \\ \text{ } & \text{ } & S_{uu}=\ 51.96108614 & \text{ } \\ \text{ } & \text{ } & C_{uu}=\ 0 & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uud}=\ 46.3561487 \\ \text{ } & \text{ } & \text{ } & C_{uud}=\ 0 \\ \text{ } & S_u=\ 45.58994896 & \text{ } & \text{ } \\ \text{ } & C_u=\ 2.41285153 & \text{ } & \text{ } \\ S=\ 40 & \text{ } & S_{ud}=S_{du}=\ 40.67225322 & \text{ } \\ C=\ 5.787711996 & \text{ } & C_{ud}=\ 4.585624746 & \text{ } \\ \text{ } & S_d=\ 35.68528077 \text{ } & \text{ } \\ \text{ } & C_d=\ 8.864829182 \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{udd}=\ 36.28501939 \\ \text{ } & \text{ } & \text{ } & C_{udd}=\ 8.714980615 \\ \text{ } & \text{ } & S_{dd}=\ 31.83598158 & \text{ } \\ \text{ } & \text{ } & C_{dd}=\ 12.79057658 & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{ddd}=\ 28.40189853 \\ \text{ } & \text{ } & \text{ } & C_{ddd}=\ 16.59810147 \\ \end{array}$

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At the node where the stock price is $S_{dd}=$ 31.83598158, the option value for the American option is in bold and is greater than the option value in the tree for the European option. This is due to the fact that early exercise is possible in the tree for the American option. When early exercise is possible, the put option value at that node is $45 –$31.83598158 = $13.16401842. As a result of the early exercise in one node, the price of the American put option is$6.0044 whereas the price of the option if early exercise is not permitted is $5.7877. Example 2 Consider Example 3 in the binomial model post #4. That example is to price a 6-month European call option in a 3-period binomial tree. The following shows the specifics of this call option. • Initial stock price is$60.
• Strike price of the call option is $55. • The stock is non-dividend paying. • The annual standard deviation of the stock return is $\sigma=$ 0.3. • The annual risk-free interest rate is $r=$ 4%. What is the price if early exercise is possible? The following is the binomial tree for the European call option from Example 3 in the previous post. $\text{ }$ Example 2 – the binomial tree and option values – European call $\text{ }$ $\displaystyle \begin{array}{llll} \displaystyle \text{Initial Price} & \text{Period 1} & \text{Period 2} & \text{Period 3} \\ \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uuu}=\ 88.39081 \\ \text{ } & \text{ } & \text{ } & C_{uuu}=\ 33.39081 \\ \text{ } & \text{ } & S_{uu}=\ 77.68226 & \text{ } \\ \text{ } & \text{ } & C_{uu}=\ 23.04770 & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uud}=\ 69.18742 \\ \text{ } & \text{ } & \text{ } & C_{uud}=\ 14.19742 \\ \text{ } & S_u=\ 68.27104 & \text{ } & \text{ } \\ \text{ } & C_u=\ 14.23394 & \text{ } & \text{ } \\ S=\ 60 & \text{ } & S_{ud}=S_{du}=\ 60.80536 & \text{ } \\ C=\ 8.26318 & \text{ } & C_{ud}=\ 6.61560 & \text{ } \\ \text{ } & S_d=\ 53.43878 \text{ } & \text{ } \\ \text{ } & C_d=\ 3.08486 \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{udd}=\ 54.15607 \\ \text{ } & \text{ } & \text{ } & C_{udd}=\ 0 \\ \text{ } & \text{ } & S_{dd}=\ 47.59506 & \text{ } \\ \text{ } & \text{ } & C_{dd}=\ 0 & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{ddd}=\ 42.39037 \\ \text{ } & \text{ } & \text{ } & C_{ddd}=\ 0 \\ \end{array}$ $\text{ }$ Observe that early exercise is optimal at none of the nodes in this binomial tree. In this example, the American call option and the European call option have the same price (when suing a 3-period binomial tree). Example 3 A 2-year American call option has the following characteristics: • Initial stock price is$75.
• Strike price of the call option is $72. • The stock pays continuous dividends at the annual rate of $\delta=$ 0.06. • The annual standard deviation of the stock return is $\sigma=$ 0.3. • The annual risk-free interest rate is $r=$ 3%. Price this call option in a 3-period binomial tree. Also compute the price for the European call with the same characteristics. $\text{ }$ Example 3 – the binomial tree and option values – American call $\text{ }$ $\displaystyle \begin{array}{llll} \displaystyle \text{Initial Price} & \text{Period 1} & \text{Period 2} & \text{Period 3} \\ \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uuu}=\ 147.2799263 \\ \text{ } & \text{ } & \text{ } & C_{uuu}=\ 75.27992628 \\ \text{ } & \text{ } & S_{uu}=\ 117.6114109 & \text{ } \\ \text{ } & \text{ } & \mathbf{C_{uu}=\ 45.61141089} & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uud}=\ 90.2367785 \\ \text{ } & \text{ } & \text{ } & C_{uud}=\ 18.2367785 \\ \text{ } & S_u=\ 93.91941129 & \text{ } & \text{ } \\ \text{ } & C_u=\ 23.94529115 & \text{ } & \text{ } \\ S=\ 60 & \text{ } & S_{ud}=S_{du}=\ 72.05920794 & \text{ } \\ C=\ 12.16262618 & \text{ } & C_{ud}=\ 7.848617166 & \text{ } \\ \text{ } & S_d=\ 57.54338237 \text{ } & \text{ } \\ \text{ } & C_d=\ 3.377832957 \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{udd}=\ 55.28707407 \\ \text{ } & \text{ } & \text{ } & C_{udd}=\ 0 \\ \text{ } & \text{ } & S_{dd}=\ 44.14987805 & \text{ } \\ \text{ } & \text{ } & C_{dd}=\ 0 & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{ddd}=\ 33.87377752 \\ \text{ } & \text{ } & \text{ } & C_{ddd}=\ 0 \\ \end{array}$ $\text{ }$ Observe that early exercise is optimal at the node where the stock price is $S_{uu}=$$117.6114109. If early exercise is not allowed, the following is the binomial tree.

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Example 3 – the binomial tree and option values – European call
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$\displaystyle \begin{array}{llll} \displaystyle \text{Initial Price} & \text{Period 1} & \text{Period 2} & \text{Period 3} \\ \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uuu}=\ 147.2799263 \\ \text{ } & \text{ } & \text{ } & C_{uuu}=\ 75.27992628 \\ \text{ } & \text{ } & S_{uu}=\ 117.6114109 & \text{ } \\ \text{ } & \text{ } & C_{uu}=\ 42.42549702 & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uud}=\ 90.2367785 \\ \text{ } & \text{ } & \text{ } & C_{uud}=\ 18.2367785 \\ \text{ } & S_u=\ 93.91941129 & \text{ } & \text{ } \\ \text{ } & C_u=\ 22.57415983 & \text{ } & \text{ } \\ S=\ 60 & \text{ } & S_{ud}=S_{du}=\ 72.05920794 & \text{ } \\ C=\ 11.57252827 & \text{ } & C_{ud}=\ 7.848617166 & \text{ } \\ \text{ } & S_d=\ 57.54338237 \text{ } & \text{ } \\ \text{ } & C_d=\ 3.377832957 \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{udd}=\ 55.28707407 \\ \text{ } & \text{ } & \text{ } & C_{udd}=\ 0 \\ \text{ } & \text{ } & S_{dd}=\ 44.14987805 & \text{ } \\ \text{ } & \text{ } & C_{dd}=\ 0 & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{ddd}=\ 33.87377752 \\ \text{ } & \text{ } & \text{ } & C_{ddd}=\ 0 \\ \end{array}$

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Practice problems

Practice problems can be found in the companion problem blog.

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$\copyright \ \ 2015 \ \text{Dan Ma}$

## The binomial option pricing model – part 4

This is post #4 on the binomial option pricing model. The purpose of post #4:

Post #4: Extend the one-period binomial option pricing calculation to more than one period.

The work in this post is heavily relying on the work in the one-period binomial option pricing model discussed in the part 1 post and in the part 2 post.

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Multi-period binomial trees

We describe how to price an option based on a multi-period binomial tree. We use a 2-period tree to anchor the discussion. Assume that the length of one period is $h$ years. Then the following 2-period binomial tree is to price a $2h$-year option (call or put). For example, if $h=$ 0.25 years, then the following binomial tree is a basis for pricing a 6-month option.

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Figure 1 – 2-period binomial tree

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The stock prices in the above binomial tree are constructed using forward prices. At the left, $S$ is the initial stock price. Then the stock prices at the end of period 1 are:

$\text{ }$
$\displaystyle S_d=S \ e^{(r-\delta) h - \sigma \sqrt{h}} \ \ \ \ \ \ \ \ \ \displaystyle S_u=S \ e^{(r-\delta) h + \sigma \sqrt{h}} \ \ \ \ \ \ \ \ \ \ \ \ \ (1)$
$\text{ }$

where $r$ is the annual risk-free interest rate, $\delta$ is the annual continuous dividend rate and $\sigma$ is the annualized standard deviation of the continuously compounded stock return. Multiplying $\sigma$ by $\sqrt{h}$ adjusts the standard deviation to make the stock return appropriate for a period of length $h$.

The stock prices at the end of period 2 are also constructed based on the idea in (1). The formula (1) takes a starting price (e.g. $S_u$) and calculates the up move, e.g. $S_{uu}$ and the down move, e.g. $S_{dd}$. The same idea in (1) can then be used to build additional periods beyond the period 2.

Because the stock prices in Figure 1 are calculated by formula (1), an up move followed by a down move leads to the same stock price as a down move followed by an up move. Thus $S_{ud}=S_{du}$ at the end of the second period. When this happens, the resulting binomial tree is called a recombining tree. When up-down move leads to a different price from a down-up move, the resulting tree is called a nonrecombining tree. When stock prices are calculated using the forward prices, the resulting binomial tree is a recombining tree.

Suppose that the binomial tree in Figure 1 models a $2h$-year option. We can compute the value of the option at each node at the end of period 2.

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Figure 2 – 2-period binomial tree with option values

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The option value is $0 if it is not advantageous for the option buyer to exercise. If it is, the option value is the difference between the stock price at expiration and the strike price. For example, for a call option, if strike price is$50 and $S_{uu}=$ $75, then $C_{uu}=$ =$25. For a put option, if strike price is $50 and if $S_{dd}=$$30, then $C_{dd}=$ $20. Once the option values at the end of the last period are known, we can calculate the option values for the preceding periods and at time 0. $\text{ }$ Figure 3 – 2-period binomial tree with option values $\text{ }$ Risk-neutral pricing is an efficient algorithm for pricing an option using a binomial tree. The option value at a given node is simply the weighted average using risk-neutral probabilities of the two option values in the next period discounted at the risk-free interest rate. The following is the risk-neutral pricing formula: $\displaystyle C^*=e^{-r h} \ [p^* \times (C^*)_u + (1-p^*) \times (C^*)_d] \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)$ where $C^*$ is the option value at a given node, and $(C^*)_u$ is the option value at the up move and (C^*)_d$ is the option value at the down move that follow the node at $C^*$. The risk-neutral probability for the up move is:

$\displaystyle p^*=\frac{e^{(r-\delta) h}-d}{u-d}$

For example, in Figure 3, the option value $C_u$ at the node for stock price $S_u$ is:

$\displaystyle C_u=e^{-r h} \ [p^* \times C_{uu} + (1-p^*) \times C_{ud}]$

Once the option values at expiration (the end of the last period in the binomial tree) are known, we can use the risk-neutral pricing formula (2) to work backward to derive the option value $C$ at the first node in the tree, which is the price of the option in question.

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The process of pricing an option using a multi-period binomial tree

The process just described can be used to price a European option based on a binomial tree of any number of periods. The process is summarized as follows:

Pricing an option using a multi-period binomial tree

1. Build a binomial tree as in Figure 1. The stock prices in this tree are relative to the forward prices as shown in formula (1).
2. Calculate the option values at the end of the last period in the tree as in Figure 2. This step is based on a comparison of the strike price and the stock prices at expiration of the option.
3. Starting from the option values at the end of the last period, work backward to calculate the option value at each node in each of the preceding periods. One way to calculate the option value at each node is to use the risk-neutral pricing formula in (2).

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Two-period examples

We demonstrate how to extend the one-period calculation to two-period through the following two examples.

Example 1
Price a one-year call option using a 2-period binomial tree. The specifics of the call option and its underlying stock are:

• Initial stock price is $60. • Strike price of the call option is$55.
• The stock is non-dividend paying.
• The annual standard deviation of the stock return is $\sigma=$ 0.3.
• The annual risk-free interest rate is $r=$ 4%.

The one-year option period is divided into two periods, making one period being 6 months. Thus $h=$ 0.5. This example is based on Example 4 in this previous post, which is about a 6-month call option with the same specifics as given above. Thus Example 1 here is Example 4 in the previous post with an additional 6-month period in the binomial tree.

Usually, in working a binomial tree problem, one tree diagram suffices. In order to make the procedure clear, we use three tree diagrams to demonstrate the three steps involved. Step 1 is to build the binomial tree. The following diagram is the result.

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Step 1: build the binomial tree (Example 1)
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$\displaystyle \begin{array}{lllll} \displaystyle \text{Initial Price} & \text{ } & \text{Period 1} & \text{ } & \text{Period 2} \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & S_{uu}=\ 95.45058 \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & S_u=\ 75.67718 & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ S= \ 60 & \text{ } & \text{ } & \text{ } & S_{ud}=\ 62.44865 \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & S_d=\ 49.51187 & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & S_{dd}=\ 40.85710 \end{array}$

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The stock prices in the above binomial tree are based on the following movement factors $u$ and $d$.

$\displaystyle u=e^{(0.04-0) 0.5 + 0.3 \sqrt{0.5}}=$ 1.261286251

$\displaystyle d=e^{(0.04-0) 0.5 - 0.3 \sqrt{0.5}}=$ 0.825197907

The following details the calculations for the stock prices:

$\displaystyle S_u=60u=$ 60 (1.261286251) = $75.67717506 $\displaystyle S_d=60d=$ 60 (0.825197907) =$49.51187441

$\displaystyle S_{uu}=u S_u$ 1.261286251 (75.67717506) = $95.45058041 $\displaystyle S_{du}=S_{ud}=d S_u$ 0.825197907 (75.67717506) =$62.44864645
$\displaystyle S_{dd}=d S_d$ 0.825197907 (49.51187441) = $40.85709513 Step 2 is to obtain the option values at expiration. For a European call option, the option value at expiration is the mximum of$0 or the stock price less the strike price. Simply compare the strike price of $55 with the stock prices at the end of the binomial tree. Any node with stock price above the strike price$55 has positive option value. The following tree shows the result.

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Step 2: add option values at expiration (Example 1)
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$\displaystyle \begin{array}{lllll} \displaystyle \text{Initial Price} & \text{ } & \text{Period 1} & \text{ } & \text{Period 2} \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & S_{uu}=\ 95.45058 \\ \text{ } & \text{ } & \text{ } & \text{ } & C_{uu}=\ 40.45058 \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & S_u=\ 75.67718 & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ S= \ 60 & \text{ } & \text{ } & \text{ } & S_{ud}=\ 62.44865 \\ \text{ } & \text{ } & \text{ } & \text{ } & C_{ud}=\ 7.44865 \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & S_d=\ 49.51187 & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & S_{dd}=\ 40.85710 \\ \text{ } & \text{ } & \text{ } & \text{ } & C_{dd}=\ 0 \end{array}$
$\text{ }$

Step 3 is to work backward from the end of the tree to the front of the tree. For example, calculate the option value at each node in period 1 by using the option values of the associated up and down nodes in period 2. We take the approach of using risk-neutral pricing described in (2). The following diagram shows the results.

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Step 3: work backward to obtain option price (Example 1)
$\text{ }$
$\displaystyle \begin{array}{lllll} \displaystyle \text{Initial Price} & \text{ } & \text{Period 1} & \text{ } & \text{Period 2} \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & S_{uu}=\ 95.45058 \\ \text{ } & \text{ } & \text{ } & \text{ } & C_{uu}=\ 40.45058 \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & S_u=\ 75.67718 & \text{ } & \text{ } \\ \text{ } & \text{ } & C_u=\ 21.76625 & \text{ } & \text{ } \\ \text{ } & \text{ } & \Delta=1.0 & \text{ } & \text{ } \\ \text{ } & \text{ } & B=- \ 53.91093 & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ S= \ 60 & \text{ } & \text{ } & \text{ } & S_{ud}=\ 62.44865 \\ C=\ 11.30954 & \text{ } & \text{ } & \text{ } & C_{ud}=\ 7.44865 \\ \Delta=0.70710 & \text{ } & \text{ } & \text{ } & \text{ } \\ B=- \ 31.11633 & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & S_d=\ 49.51187 & \text{ } & \text{ } \\ \text{ } & \text{ } & C_d=\ 3.26482 & \text{ } & \text{ } \\ \text{ } & \text{ } & \Delta=0.34498 & \text{ } & \text{ } \\ \text{ } & \text{ } & B=- \ 13.81577 & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & S_{dd}=\ 40.85710 \\ \text{ } & \text{ } & \text{ } & \text{ } & C_{dd}=\ 0 \end{array}$
$\text{ }$

As mentioned above, to calculate the option values, we use risk-neutral probabilities:

$\displaystyle u=e^{(0.04-0) 0.5 + 0.3 \sqrt{0.5}}=$ 1.261286251

$\displaystyle d=e^{(0.04-0) 0.5 - 0.3 \sqrt{0.5}}=$ 0.825197907

$\displaystyle p^*=\frac{e^{(0.04-0)0.5}-0.825197907}{1.261286251-0.825197907}=$ 0.447164974

$\displaystyle 1-p^*=$ 0.552835026

The following computes the option values:

$C_u=e^{-0.04 (0.5)} [ p* (40.45058041)+(1-p^*)(7.44864645)]=$ $21.76624801 $C_d=e^{-0.04 (0.5)} [ p* (7.44864645)+(1-p^*)(0)]=$$3.264820056

$C=e^{-0.04 (0.5)} [ p* (21.76624801)+(1-p^*)(3.264820056)]=$ 11.30954269 The diagram in Step 3 also shows the replicating at each node. For example, the replicating portfolio at the node for $C_{u}$ is computed as follows: $\displaystyle \Delta=e^{0(0.5)} \ \frac{C_{uu}-C_{ud}}{S_{uu}-S_{ud}}=\frac{40.45058041-7.44864645}{95.45058041-62.44864645}=$ 1.0 \displaystyle \begin{aligned} B&=e^{-0.04(0.5)} \ \frac{u C_{ud}-d C_{uu}}{u-d} \\&=e^{-0.02} \ \frac{1.261286251 (7.44864645)-0.825197907 (40.45058041)}{1.261286251-0.825197907} \\&=- \ 53.91093 \end{aligned} ___________________________________________________________________________________ Example 2 This is Example 5 in this previous post. Example 5 in that post is a 3-month put option. We now price the same 3-month put option using a 2-period binomial tree. Thus the 3-month option period is divided into two periods. The following gives the specifics of this put option: • Initial stock price is40.
• Strike price of the put option is $45. • The stock is non-dividend paying. • The annual standard deviation of the stock return is $\sigma=$ 0.3. • The annual risk-free interest rate is $r=$ 5%. We carry out the same three steps as in Example 1. The following diagram captures the results of all three steps. $\text{ }$ Example 2: binomial tree for pricing put option $\text{ }$ $\displaystyle \begin{array}{lllll} \displaystyle \text{Initial Price} & \text{ } & \text{Period 1} & \text{ } & \text{Period 2} \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & S_{uu}=\ 50.07448 \\ \text{ } & \text{ } & \text{ } & \text{ } & C_{uu}=\ 0 \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & S_u=\ 44.75466 & \text{ } & \text{ } \\ \text{ } & \text{ } & C_u=\ 2.35281 & \text{ } & \text{ } \\ \text{ } & \text{ } & \Delta=-0.46983 & \text{ } & \text{ } \\ \text{ } & \text{ } & B=- \ 23.37970 & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ S= \ 40 & \text{ } & \text{ } & \text{ } & S_{ud}=\ 40.50314 \\ C=\ 5.56462 & \text{ } & \text{ } & \text{ } & C_{ud}=\ 4.49686 \\ \Delta=-0.72087 & \text{ } & \text{ } & \text{ } & \text{ } \\ B=\ 34.39932 & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & S_d=\ 36.20016 & \text{ } & \text{ } \\ \text{ } & \text{ } & C_d=\ 8.51947 & \text{ } & \text{ } \\ \text{ } & \text{ } & \Delta=-1 & \text{ } & \text{ } \\ \text{ } & \text{ } & B=- \ 44.71963 & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{ } & S_{dd}=\ 32.76128 \\ \text{ } & \text{ } & \text{ } & \text{ } & C_{dd}=\ 12.23872 \end{array}$ $\text{ }$ Note that the put option calculated in Example 5 in this previous post using one binomial period is$5.3811 whereas the put option price from a 2-period binomial tree here is $5.56462. It is not uncommon for binomial option prices to fluctuate when the number of periods $n$ is small. When $n$ is large, the binomial price will stabilize. Note that the option period is 3-month long (a quarter of a year). Thus one period is $h=$ 0.25/2 = 0.125 of a year. To build the binomial tree, the following shows the calculation for the stock prices $S_d$ and $S_{du}$. $\displaystyle u=e^{(0.05-0) 0.125 + 0.3 \sqrt{0.125}}=e^{0.1625}=$ 1.118866386 $\displaystyle d=e^{(0.05-0) 0.125 - 0.3 \sqrt{0.125}}=e^{-0.1375}=$ 0.905003908 $\displaystyle S_d=40d=$ 40 (0.905003908) =$36.20015632

$\displaystyle S_{ud}=S_{du}=S_d \ d=$ 36.20015632 (1.118866386) = $40.50313807 As in Example 1, we perform risk-neutral pricing. The following shows the calculation of the option values. $\displaystyle p^*=\frac{e^{(0.05-0)0.25}-0.87153435}{1.176448318-0.87153435}=$ 0.462570155 $\displaystyle 1-p^*=$ 0.537429845 $C_u=e^{-0.05 (0.125)} [ p* (0)+(1-p^*)(4.496861938)]=$$2.352809258

$C_d=e^{-0.05 (0.125)} [ p* (4.496861938)+(1-p^*)(12.23871707)]=$ $8.519470762 $C=e^{-0.05 (0.125)} [ p* (2.352809258)+(1-p^*)(8.519470762)]=$$5.564617421

The diagram in Example 2 also shows the replicating at each node. For example, the replicating portfolio at the node for $C_{d}$ is computed as follows:

$\displaystyle \Delta=e^{0(0.125)} \ \frac{C_{du}-C_{dd}}{S_{du}-S_{dd}}=\frac{4.496861938-12.23871707}{40.50313806-32.76128293}=$ -1.0

\displaystyle \begin{aligned} B&=e^{-0.05(0.125)} \ \frac{u C_{dd}-d C_{du}}{u-d} \\&=e^{-0.00625} \ \frac{1.118866386 (12.23871707)-0.905003908 (4.496861938)}{1.118866386-0.905003908} \\&=- \ 44.71962708 \end{aligned}

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Binomial trees with more than two periods

Since one or two-period binomial trees are unlikely to be accurate model of stock price movements, option prices based on the binomial model with one or two periods are unlikely to be accurate. It is then necessary to use more periods in the binomial tree, i.e. divide the time to expiration into more periods to create more realistic model of stock price movements. Therefore realistic applications of the binomial option pricing model require the use of software.

Another point we would like to make is that using more periods in the binomial tree requires no new concepts or new methods. The same three steps described above are used – build the binomial tree, calculate the option values at expiration and work backward to derive the option price. The calculation at each node still uses the same one-period binomial option formulas. It is just that there are more periods to calculate. Hence realistic binomial option pricing is a job that should be done by software. To conclude this post, we present an example using a three-period binomial tree.

Example 3
Like Example 1 above, this example is based on Example 4 in this previous post. Example 4 in that post is to price a 6-month call option. In this example, we price the same call options using a 3-period binomial tree. All other specifics of the call option and the underlying stock remain the same. They are repeated here:

• Initial stock price is $60. • Strike price of the call option is$55.
• The stock is non-dividend paying.
• The annual standard deviation of the stock return is $\sigma=$ 0.3.
• The annual risk-free interest rate is $r=$ 4%.

Now one period is 2-month long. Hence in the calculation $h=$ 2/12 = 0.16667. The results of the 3-period binomial calculation are show in the following two trees. The first one displays the stock prices and the option values. The second one displays the replicating portfolios (the hedge ratio $\Delta$ and the amount of borrowing $B$) at each node.

$\text{ }$

Example 3 – the binomial tree and option values
$\text{ }$
$\displaystyle \begin{array}{llll} \displaystyle \text{Initial Price} & \text{Period 1} & \text{Period 2} & \text{Period 3} \\ \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uuu}=\ 88.39081 \\ \text{ } & \text{ } & \text{ } & C_{uuu}=\ 33.39081 \\ \text{ } & \text{ } & S_{uu}=\ 77.68226 & \text{ } \\ \text{ } & \text{ } & C_{uu}=\ 23.04770 & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{uud}=\ 69.18742 \\ \text{ } & \text{ } & \text{ } & C_{uud}=\ 14.19742 \\ \text{ } & S_u=\ 68.27104 & \text{ } & \text{ } \\ \text{ } & C_u=\ 14.23394 & \text{ } & \text{ } \\ S=\ 60 & \text{ } & S_{ud}=S_{du}=\ 60.80536 & \text{ } \\ C=\ 8.26318 & \text{ } & C_{ud}=\ 6.61560 & \text{ } \\ \text{ } & S_d=\ 53.43878 \text{ } & \text{ } \\ \text{ } & C_d=\ 3.08486 \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{udd}=\ 54.15607 \\ \text{ } & \text{ } & \text{ } & C_{udd}=\ 0 \\ \text{ } & \text{ } & S_{dd}=\ 47.59506 & \text{ } \\ \text{ } & \text{ } & C_{dd}=\ 0 & \text{ } \\ \text{ } & \text{ } & \text{ } & S_{ddd}=\ 42.39037 \\ \text{ } & \text{ } & \text{ } & C_{ddd}=\ 0 \\ \end{array}$

$\text{ }$

$\text{ }$

Example 3 – Replicating portfolios
$\text{ }$
$\displaystyle \begin{array}{llll} \displaystyle \text{Initial Price} & \text{Period 1} & \text{Period 2} & \text{Period 3} \\ \text{ } & \text{ } & \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{N/A} \\ \text{ } & \text{ } & \text{ } & \text{N/A} \\ \text{ } & \text{ } & \Delta=1 & \text{ } \\ \text{ } & \text{ } & B=-\ 54.63456 & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{N/A} \\ \text{ } & \text{ } & \text{ } & \text{N/A} \\ \text{ } & \Delta=0.97364 & \text{ } & \text{ } \\ \text{ } & B=-\ 52.23779 & \text{ } & \text{ } \\ \Delta=0.75168 & \text{ } & \Delta=0.94386 & \text{ } \\ B=-\ 36.83749 & \text{ } & B=-\ 50.77586 & \text{ } \\ \text{ } & \Delta=0.50079 \text{ } & \text{ } \\ \text{ } & B=-\ 23.67681 \text{ } & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{N/A} \\ \text{ } & \text{ } & \text{ } & \text{N/A} \\ \text{ } & \text{ } & \Delta=0 & \text{ } \\ \text{ } & \text{ } & B=\ 0 & \text{ } \\ \text{ } & \text{ } & \text{ } & \text{N/A} \\ \text{ } & \text{ } & \text{ } & \text{N/A} \\ \end{array}$

$\text{ }$

The call option price using one-period tree in Example 4 in the previous post is $9.06302. The 3-period option price using a 3-period tree is$8.26318. Once again, there is no need to be alarmed. Binomial option prices can wildly fluctuate when the number of periods is small. The example here is only meant to illustrate the calculation in binomial option model.

Just to be clear on the process, the stock prices in the upper two nodes in the third period are calculated as follows:

$\displaystyle u=e^{(0.04-0) \frac{1}{6} + 0.3 \sqrt{\frac{1}{6}}}=$ 1.137850725

$\displaystyle d=e^{(0.04-0) \frac{1}{6} - 0.3 \sqrt{\frac{1}{6}}}=$ 0.890646371

$\displaystyle S_{uuu}=S_{uu} \ u=$ 77.68225631 (1.137850725) = $88.39081165 $\displaystyle S_{uud}=S_{uu} \ d=$ 77.68225631 (0.890646371) =$69.18741967

The option value at the node $S_{uu}$ is calculated as follows using risk-neutral probabilities:

$\displaystyle p^*=\frac{e^{(0.05-0)0.25}-0.87153435}{1.176448318-0.87153435}=$ 0.462570155

$\displaystyle 1-p^*=$ 0.537429845

$C_u=e^{-0.05 (0.125)} [ p* (0)+(1-p^*)(4.496861938)]=$ $2.352809258 $C_d=e^{-0.05 (0.125)} [ p* (4.496861938)+(1-p^*)(12.23871707)]=$$8.519470762

$C=e^{-0.05 (0.125)} [ p* (2.352809258)+(1-p^*)(8.519470762)]=$ $5.564617421 ___________________________________________________________________________________ Practice problems For practice problems on how to calculate price of European option using multiperiod binomial tree, go here in the practice problem companion blog. ___________________________________________________________________________________ $\copyright \ \ 2015 \ \text{Dan Ma}$ ## The binomial option pricing model – part 1 This is post #1 on the binomial option pricing model. Even though this is post #1, there are two previous posts with examples to illustrate how to price options using the one-period binomial pricing model (example of call and example of put). The purpose of post #1: Post #1: Describe the option pricing formulas in the one-period binomial model. ___________________________________________________________________________________ The one-period binomial option pricing model We first consider the pricing of options on stock. The most important characteristic of the binomial option pricing model is that over a period of time, the stock price is assumed to follow a binomial distribution, i.e. the price of the stock can only take on one of two values – an upped value and a downed value. In this post, we describe how to price an option on a stock using this simplifying assumption of stock price movement. Consider a stock with the following characteristics: • The current share price is $S$. • If the stock pays dividends, we assume the dividends are paid at an annual continuous rate at $\delta$. • At the end of a period of length $h$ (in years), the share price is either $S_h=uS$ or $S_h=dS$, where $u$ is the up factor and $d$ is the down factor. The factor $u$ can be interpreted as one plus the rate of capital gain on the stock if the stock goes up. The factor $d$ can be interpreted as one plus the rate of capital loss if the stock goes down. • If $\delta>0$, the end of period share price is $S_h=uS e^{\delta h}$ or $S_h=dS e^{\delta h}$. This is to reflect the gains from reinvesting the dividends. Of course if $\delta=0$, the share prices revert back to the previous bullet point. The end of period stock prices are shown in the following diagram, which is called a binomial tree since it depicts the 2-state stock price at the end of the option period. $\text{ }$ Figure 1 – binomial tree $\text{ }$ Now consider a European option (either call or put) on the stock described above. When the stock goes up, we use $C_u$ to represent the value of the option. When the stock goes down, we use $C_d$ to represent the value of the option. The following is the binomial tree for the value of the option. $\text{ }$ Figure 2 – option value tree $\text{ }$ Replicating Portfolio The key idea to price the option is to create a portfolio consisting of $\Delta$ shares of the stock and the amount $B$ in lending. At time 0, the value of this portfolio is $C=\Delta S + B$. At time $h$ (the end of the option period), the value of the portfolio is $\text{ }$ Time $h$ value of the replicating portfolio $\displaystyle \text{ } \left\{\begin{matrix} \displaystyle \Delta \times (dS \ e^{\delta h}) + B \ e^{r h}&\ \ \ \ \ \ \text{(when stock price goes down)}& \\ \text{ }&\text{ } \\ \Delta \times (uS \ e^{\delta h}) + B \ e^{r h}&\ \ \ \ \ \ \text{(when stock price goes up)} \end{matrix}\right.$ $\text{ }$ This portfolio is supposed to replicate the same payoff as the value of the option. By equating the portfolio payoff with the option payoff, we obtain the following linear equations. $\text{ }$ $\displaystyle \text{ } \left\{\begin{matrix} \displaystyle \Delta \times (dS \ e^{\delta h}) + B \ e^{r h}=C_d&\ \ \ \ \ \ \text{ }& \\ \text{ }&\text{ } \\ \Delta \times (uS \ e^{\delta h}) + B \ e^{r h}=C_u&\ \ \ \ \ \ \text{ } \end{matrix}\right.$ $\text{ }$ There are two unknowns in the above two equations. All the other items – stock price $S$, dividend rate $\delta$, and risk-free interest rate $r$ – are known. Solving for the two unknowns $\Delta$ and $B$, we obtain: $\text{ }$ $\displaystyle \Delta=e^{-\delta h} \ \frac{C_u-C_d}{S(u-d)} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (1)$ $\text{ }$ $\displaystyle B=e^{-r h} \ \frac{u \ C_d-d \ C_u}{u-d} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)$ $\text{ }$ Once the replication portfolio of $\Delta$ shares and $B$ in lending is determined, the price of the option (the value at time 0) is: $\text{ }$ $C=\Delta S + B \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (3)$ $\text{ }$ After plugging in (1) and (2) into (3), the option price formula becomes: $\text{ }$ $\displaystyle C=\Delta S + B=e^{-r h} \biggl(C_u \ \frac{e^{(r-\delta) h}-d}{u-d} +C_d \ \frac{u-e^{(r-\delta) h}}{u-d} \biggr) \ \ \ \ \ \ \ \ \ (4)$ $\text{ }$ The price of the option described above is $C$, either given by formula (3) or formula (4). One advantage of formula (4) is that it gives the direct calculation of the option price without knowing $\Delta$ and $B$. Of course, if the goal is to create a synthetic option for the purpose of hedging or risk management, it will be necessary to know the make up of the replicating portfolio. The $\Delta$ calculated in (1) is also called the hedge ratio and is examined in greater details in in this subsequent post. ___________________________________________________________________________________ Examples Example 1 Let’s walk through a quick example to demonstrate how to apply the above formulas. Suppose that the future prices for a stock are modeled with a one-period binomial tree with $u=$ 1.3 and $d=$ 0.8 and having a period of 6 months. The current price of the stock is$50. The stock pays no dividends. The annual risk-free interest rate is $r=$ 4%.

• Determine the price of a European 55-strike call option on this stock that will expire in 6 months.
• Determine the price of a European 45-strike put option on this stock that will expire in 6 months.

The two-state stock prices are $65 and$40. The two-state call option values at expiration are $10 and$0. Apply (1) and (2) to obtain the replicating portfolio and then the price of the call option.

$\text{ }$
$\displaystyle \Delta=\frac{10-0}{65-40}=\frac{10}{25}=$ 0.4

$\displaystyle B=e^{-0.04(0.5)} \ \frac{1.3(0)-0.8(10)}{1.3-0.8}=-16 e^{-0.02}=$ -$15.68317877 The replicating portfolio consists of holding 0.4 shares and borrowing$15.68317877.

Call option price = $50 \Delta+B=$ $4.316821227 $\text{ }$ The 2-state put option values at expiration are$0 and $5. Now apply (1) and (2) and obtain: $\text{ }$ $\displaystyle \Delta=\frac{0-5}{65-40}=\frac{-5}{25}=-0.2$ $\displaystyle B=e^{-0.04(0.5)} \ \frac{1.3(5)-0.8(0)}{1.3-0.8}=13 e^{-0.02}=$$12.74258275

The replicating portfolio consists of shorting 0.2 shares and lending $12.74258275. Put option price = $50 \Delta+B=$$2.742582753

$\text{ }$

Example 1 is examined in greater details in this subsequent post.

More Examples
Two more examples are in these previous posts:

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What to do if options are mispriced

What if the observed price of an option is not the same as the theoretical price? In other words, what if the price of a European option is not given by the above formulas? Because we can always hold stock and lend to replicate the payoff of an option, we can participate in arbitrage when an option is mispriced by buying low and selling high. The idea is that if an option is underpriced, then we buy low (the underpriced option) and sell high (the corresponding synthetic option, i.e. the replicating portfolio). On the other hand, if an option is overpriced, then we buy low (the synthetic option) and sell high (the overpriced option). Either case presents risk-free profit. We demonstrate with the options in Example 1.

Example 2

• Suppose that the price of the call option in Example 1 is observed to be $4.00. Describe the arbitrage. • Suppose that the price of the call option in Example 1 is observed to be$4.60. Describe the arbitrage.

For the first scenario, we buy low (the option at $4.00) and sell the synthetic option at the theoretical price of$4.316821227. Let’s analyze the cash flows in the following table.

$\text{ }$

Table 1 – Arbitrage opportunity when call option is underpriced

$\left[\begin{array}{llll} \text{Expiration Cash Flows} & \text{ } & \text{Share Price = } \ 40 & \text{Share Price = } \ 65 \\ \text{ } & \text{ } \\ \text{Sell synthetic call} & \text{ } & \text{ } & \text{ } \\ \ \ \ \ \text{Short 0.4 shares} & \text{ } & - \ 16 & - \ 26 \\ \ \ \ \ \text{Lend } \ 15.683 & \text{ } & + \ 16 & + \ 16 \\ \text{ } & \text{ } \\ \text{Buy call } & \text{ } & \ \ \ 0 & \ \ \ 10 \\ \text{ } & \text{ } \\ \text{Total payoff} & \text{ } & \text{ } \ \ 0 & \ \ \ 0 \end{array}\right]$

$\text{ }$

The above table shows that the buy low sell high strategy produces no loss at expiration of the option regardless of the share prices at the end of the option period. But the payoff at time 0 is certain: $4.316821227 –$4.00 = $0.316821227. For the second scenario, we still buy low and sell high. This time, buy low (the synthetic call option at$4.316821227) and sell high (the call option at the observed price of $4.60). Let’s analyze the cash flows in the following table. $\text{ }$ Table 2 – Arbitrage opportunity when call option is overpriced $\left[\begin{array}{llll} \text{Expiration Cash Flows} & \text{ } & \text{Share Price = } \ 40 & \text{Share Price = } \ 65 \\ \text{ } & \text{ } \\ \text{Buy synthetic call} & \text{ } & \text{ } & \text{ } \\ \ \ \ \ \text{Long 0.4 shares} & \text{ } & + \ 16 & + \ 26 \\ \ \ \ \ \text{Borrow } \ 15.683 & \text{ } & - \ 16 & - \ 16 \\ \text{ } & \text{ } \\ \text{Buy call } & \text{ } & \ \ \ 0 & - \ 10 \\ \text{ } & \text{ } \\ \text{Total payoff} & \text{ } & \text{ } \ \ 0 & \ \ \ 0 \end{array}\right]$ $\text{ }$ The above table shows that the buy low sell high strategy produces no loss at expiration of the option regardless of the share prices at the end of the option period. But the payoff at time 0 is certain:$4.60 – $4.316821227 =$0.283178773.

These two examples show that if the option price is anything other than the theoretical price, there are arbitrage opportunities and there is risk-free profit to be made.

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How to construct a binomial tree

In the binomial tree in Figure 1, we assume that the share price at expiration is obtained by multiplying the original share price by the movement factors of $u$ and $d$. The binomial tree in Figure 1 may give the impression that the choice of the movement factors $u$ and $d$ is arbitrary as long as the up factor is greater than 1 and the down factor is below 1. In the next post, we show that $u$ and $d$ have to satisfy the following relation, else there will be arbitrage opportunities.

$\displaystyle d < e^{(r-\delta) h} < u \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (5)$

Thus the choice of $u$ and $d$ cannot be entirely arbitrary. In particular the relation (5) shows that the future stock prices have to revolve around the forward price.

$\displaystyle dS < Se^{(r-\delta) h} < uS \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (6)$

The purpose pf the factors $u$ and $d$ in the binomial tree is to incorporate uncertainty of the stock prices. In light of (6), we can set $u$ and $d$ by applying some volatility adjustment to $e^{(r-\delta) h}$. We can use the following choice of $u$ and $d$ to model the stock price evolution.

$\displaystyle u = e^{(r-\delta) h \ + \ \sigma \sqrt{h}}$

$\displaystyle d = e^{(r-\delta) h \ - \ \sigma \sqrt{h}} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (7)$

where

$\sigma$ is the annualized standard deviation of the continuously compounded stock return,

$\sigma \sqrt{h}$ is the standard deviation of the continuously compounded stock return over a period of length $h$.

The standard deviation $\sigma$ measures how certain we are that the stock return will be close to the expected return. There will be a greater chance of a return far from the expected return if the stock has a higher $\sigma$. If $\sigma=0$, then there is no uncertainty about the future stock prices. The formula (7) shows that when $\sigma=0$, the future stock price is precisely the forward price on the stock. When the binomial tree is constructed using (7), the tree will be called a forward tree.

A note on calculation. If a problem does not specific $u$ and $d$ but assume a standard deviation of stock return $\sigma$, then assume that the binomial tree is the forward tree. We now use a quick example to demonstrate how to price an option using the forward tree.

Example 3
Everything is the same as Example 1 except that the up and down stock prices are constructed using the volatility $\sigma=$ 30% (the standard deviation $\sigma$). The following calculates the stock prices at expiration of the option.

$\displaystyle uS = 50 \ e^{(0.04-0) 0.5 \ + \ 0.3 \sqrt{0.5}}=$ $63.06431255 $\displaystyle dS = 50 \ e^{(0.04-0) 0.5 \ - \ 0.3 \sqrt{0.5}}=$$41.25989534

$\displaystyle u=\frac{63.06431255}{50}=$ 1.261286251

$\displaystyle d=\frac{41.25989534}{50}=$ 0.825197907

Using formulas (1), (2) and (3), the following shows the replicating portfolio and the call option price. Note that the binomial tree is based on a different assumption than that in Example 1. The option price is thus different than the one in Example 1.

$\text{ }$

$\displaystyle \Delta=\frac{8.064312548-0}{63.06431255-41.25989534}=$ 0.369847654

$\displaystyle B=e^{-0.04(0.5)} \ \frac{1.261286251(0)-0.825197907(8.064312548)}{1.261286251-0.825197907}=$ –$14.95770971 The replicating portfolio consists of holding 0.369847654 shares and borrowing$14.95770971.

Call option price = $50 \Delta+B=$ $3.534672982 $\text{ }$ The following shows the calculation for the put option. $\text{ }$ $\displaystyle \Delta=\frac{0-3.740104659}{63.06431255-41.25989534}=$ -0.171529678 $\displaystyle B=e^{-0.04(0.5)} \ \frac{1.261286251(3.740104659)-0.825197907(0)}{1.261286251-0.825197907}=$$10.60320232

The replicating portfolio consists of shorting 0.171529678 shares and lending $10.60320232. Put option price = $50 \Delta+B=$$2.026718427

$\text{ }$

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More examples

We present two more examples in illustrating the calculation in the one-period binomial option model where the stock prices are modeled by a forward tree.

Example 4
The stock price follows a 6-month binomial tree with initial stock price $60 and $\sigma=$ 0.3. The stock is non-dividend paying. The annual risk free interest rate is $r=$ 4%. What is the price of a 6-month 55-strike call option? Determine the replicating portfolio that has the same payoff as this call option. We will use risk-neutral probabilities to price the option. $\displaystyle uS = 60 \ e^{(0.04-0) 0.5 \ + \ 0.3 \sqrt{0.5}}=$$75.67717506

$\displaystyle dS = 60 \ e^{(0.04-0) 0.5 \ - \ 0.3 \sqrt{0.5}}=$ $49.51187441 $\displaystyle C_u=$ 75.67717506 – 55 = 20.67717506 $\displaystyle C_d=$ 0 $\displaystyle u=\frac{75.67717506}{60}=$ 1.261286251 $\displaystyle d=\frac{49.51187441}{60}=$ 0.825197907 $\displaystyle p^*=\frac{e^{(0.04-0) 0.5} - 0.825197907}{1.261286251 - 0.825197907}=$ 0.447164974 $\displaystyle 1-p^*=$ 0.552835026 $\displaystyle C=(p^* \times C_u + (1-p^*) \times C_d) e^{-0.02}=$ 9.063023234 $\text{ }$ $\displaystyle \Delta=\frac{20.67717506-0}{75.67717506-49.51187441}=$ 0.790251766 $\displaystyle B=e^{-0.04(0.5)} \ \frac{1.261286251(0)-0.825197907(20.67717506)}{1.261286251-0.825197907}=$ –$38.35208275

The replicating portfolio consists of holding 0.79025 shares and borrowing $38.352. $\text{ }$ Example 5 The stock price follows a 3-month binomial tree with initial stock price$40 and $\sigma=$ 0.3. The stock is non-dividend paying. The annual risk free interest rate is $r=$ 5%. What is the price of a 3-month 45-strike put option on this stock? Determine the replicating portfolio that has the same payoff as this put option.

The calculation is calculated as in Example 3.

$\displaystyle uS = 40 \ e^{(0.05-0) 0.25 \ + \ 0.3 \sqrt{0.25}}=$ $47.05793274 $\displaystyle dS = 40 \ e^{(0.05-0) 0.25 \ - \ 0.3 \sqrt{0.25}}=$$34.861374

$\displaystyle C_u=$ 0

$\displaystyle C_d=$ 45 – 34.861374 = $10.138626 $\displaystyle u=\frac{47.05793274}{40}=$ 1.176448318 $\displaystyle d=\frac{34.861374}{40}=$ 0.87153435 $\displaystyle p^*=\frac{e^{(0.05-0) 0.25} - 0.87153435}{1.176448318 - 0.87153435}=$ 0.462570155 $\displaystyle 1-p^*=$ 0.537429845 $\displaystyle C=(p^* \times C_u + (1-p^*) \times C_d) e^{-0.0125}=$ 5.381114117 $\text{ }$ $\displaystyle \Delta=\frac{0-10.138626}{47.05793274-34.861374}=$ -0.831269395 $\displaystyle B=e^{-0.05(0.25)} \ \frac{1.176448318(10.138626)-0.87153435(0)}{1.176448318 - 0.87153435}=$$38.63188995

The replicating portfolio consists of shorting 0.831269395 shares and lending \$38.63188995.

$\text{ }$

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Remarks

The discussion in this post is only the beginning of the binomial pricing model. The concepts and the formulas for the one-period binomial option model are very important. The one-period model may seem overly simplistic (or even unrealistic). One way to make it more realistic is to break up the one-period into multiple smaller periods and thus produce a more accurate option price. The calculation for the multi-period binomial model is still based on the calculation for the one-period model. Before moving to the multi-period model, we discuss the one-period model in greater details to gain more understanding of the one-period model.

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Practice problems

Practice Problems
Practice problems can be found in the companion problem blog via the following links:

basic problem set 1

basic problem set 2

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$\copyright \ \ 2015 \ \text{Dan Ma}$