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1、CHAPTER 23 OPTION CONTRACTS Answers to Questions 1.A long straddle consists of a long call and a long put on the same stock and profits from dramatic price movement by the stock. A short straddle involves the sale of a call and a put on the same stock and profits from little or no stock price change
2、. Investors going long would anticipate volatility in excess of that discounted by the options prices while investors going short would expect volatility below that already discounted. Since volatility enhances option prices, long straddles would tend to pay higher premiums for more volatile options
3、, whereas short straddles must accept lower premiums for less volatile options. 2.A range forward is actually an option strategy that combines a long call and a short put (or vice versa) through a costless transaction. Because the options will not have the same striking price, the combination is cla
4、ssified as a range forward as opposed to an actual forward, created by combining long and short options with the same striking price. It is fair to view actual forwards as a special case (or zero-cost version) of range forwards. 3.CFA Examination II (1993) Call options give the owner the right, but
5、not the obligation, to purchase SFrs for a pre- specified amount of domestic currency. Purchasing an at-the-money call option would guarantee the current exchange rate over the life of the option. If the SFr declines in value, the call will not be exercised since francs can be purchased more cheaply
6、 in the open market and redeeming the bond issue will be less costly. Contrasting characteristics: (1) Currency options are traded worldwide and enjoy a liquid market. (2) Exchange-traded currency option contracts have standard amounts, maturities, etc. (3) Over-the-counter options could be tailored
7、 to meet Michelles needs. (4) The initial cash outflow would be the premium. (5) The use of options preserves the ability to profit. (6) No counterparty credit risk. (7) Must roll to match year obligation. Currency forward contracts commit the seller to deliver the specified amount of currency to th
8、e buyer on a specified future date at a fixed price. A short position in a forward contract requires delivery. Contrasting characteristics: (1) The market for forward contracts is over-the-counter and sometimes may not be as liquid as option or futures market. (2) Forward contracts may be custom-des
9、igned for specific applications. (3) Cash does not change hands until a forward contract is settled. (4) Counterparty credit risk. (5) Can best match 5-year obligation. Currency futures are like forward contracts except the gain or loss on the contract is settled daily under the supervision of an or
10、ganized exchange. A short position in the futures requires either offset or delivery at expiration. Contrasting characteristics: (1) Futures are traded in standardized contracts and are highly liquid. (2) Cash is required for daily settlement. (3) A margin account is required. (4) Management and adm
11、inistration costs are higher than with a forward or option contract. (5) No counterparty credit risk. (6) Must roll to match year obligation. 4.CFA Examination III (1991) The other three factors affecting the value of call options and the ways that changes in them affect value are: (1). Increases in
12、 underlying stock volatility. A call cannot be worth less than zero no matter how far the stock price falls, but rising stock prices can increase the calls value without limit. Therefore, the wider the range within which a stocks price can fluctuate (i.e. the greater its volatility), the greater the
13、 chance that the option will expire in-the-money, the higher the expected payoff from owning it, and the higher its value. A wider range of probable future prices on the underlying stock increases the probability of higher payoffs in general but because the calls value cannot decline below zero does
14、 not symmetrically increase the probability of lower payoffs. (2). The risk-free interest rate. Call value increase with increases in interest rates (given constant stock prices) because higher interest rates make the ownership of call options more attractive. The call owner does not pay for the sto
15、ck until the option is exercised; its owner can, therefore, take advantage of the time value of money by investing free interest rate increases the time value benefit to the call owner, increasing the value of the call option. (3). The exercise price of the option. Call values decrease with increase
16、s in the exercise price. When a call option is exercised, the payoff is the difference between the stock price at the time of exercise and the exercise (or strike) price. A higher exercise price decreases the expected payoff from the call, thus reducing the options value. 5.Put-call parity indicates
17、 that a long position in a stock combined with being short a call and long a put (with the same strike price) is a risk-free investment. In other words, no matter what the stock price at expiration, the payoff will be the same. Consequently, any investment in this portfolio should earn the risk-free
18、 return. The three-step process for valuing options is to (1).Determine a distribution of future stock prices, (2).Calculate the cash flows from the option at the future prices, and (3).Discount these expected cash flows to the present at the risk-free rate. It is this final step that is relevant. C
19、ash flows can be discounted at the risk-free rate because of the riskless replicating portfolio strategy. 6.The Black-Scholes model is derived by showing how a portfolio of the underlying asset and risk-free bonds can be created that exactly mimics the price of an option. This involves taking a long
20、 position in the underlying asset to replicate a call option. For currency options, the underlying asset can be thought of as risk-free deposits (bonds) in the foreign currency. So, just as stocks pay dividends, foreign deposits pay interest. Therefore, we can just substitute the foreign risk-free r
21、ate for the dividend yield when pricing options on currency. 7.If there are no transaction costs, it is only rational to exercise a call option early, immediately before a dividend payment. This is because it will always be more profitable to sell the option and buy the stock in the open market rath
22、er than exercise the option, unless there is a dividend. When a firm pays a dividend, the price of the stock usually declines by approximately the amount of the dividend. This has two impacts on the option holder. First, the decrease in the stock price decreases the value of his option. Second, the
23、investor does not get the dividend payment unless he actually has exercised the option and owns the stock. Consequently, if the value of the dividend is greater than C+K-S, it will be beneficial to exercise early. Note, this is most likely to happen for every in-the-money options. For put options, i
24、t may be the case that the interest that could be earned on the proceeds from early exercise is greater than the intrinsic option value (P+S-K), so early proceeds from early exercise is optimal. Using similar logic as above, puts should always be exercised immediately after a dividend (on a dividend
25、 paying stock) because the stocks price declines after a dividend, thus making the put more valuable. 8.In the Black-Scholes model, the expected future value of a stock is a function of the risk- free interest rate and the dividend yield. As long as the risk-free rate is greater than the dividend yi
26、eld, the future expected value will be greater than todays price. The longer the time period, the higher the expected price. So, as time to expiration increases, there are two opposing forces on the value of a European put. First, the increased time to expiration increases the chances of the option
27、being more in-the-money. This increases put value. Second, the higher expected price at expiration decreases the expected value of the puts payoff at expiration and, therefore, decreases the put value. Depending on which of these two effects is larger, the put may increase or decrease in price with
28、an increase in time to expiration. For a European call option, these two effects work in the same direction, since an increase in expected future price increases the value of a call. Hence, an increase in the time to expiration always increases the value of a European call. 9.Since the price of an o
29、ption is positively related to volatility, “buying low vol and selling high vol” is the same as the idea of “buy low, sell high” for any risky asset if the other parameters that affect option prices are fixed. The other factors that affect option prices can be effectively neutralized by holding the
30、appropriate portfolio of options and the underlying asset (beyond the scope of this question). If this is the case, then the risky asset is volatile itself, not the underlying foreign currency. 10.A decrease in security volatility will cause an increase in both the call and put option values. For ex
31、ample, when the volatility of the underlying assets price decreases, the call option becomes less valuable since this decreases the probability that the option will be deeper in the money at expiration (a similar scenario is also true for the put option). 11. On October 19, 1987, implied volatilitie
32、s sky-rocketed. The jump in implied volatility increased the value of call options more than enough to offset the negative impact of the in the index level. .CHAPTER 23 Answers to Problems 1.CFA Examination III (1987) 1(a). Cu - Cd 20 - 0 Hedge ratio = = = 0.5 uS dS 120 80 (1 + r) - d (1 + 0.1) - 0.
33、0 Implied probability (p) = = = 0.75 u d 1.2 0.8 pCu + (1 p)Cd .75(20)+.25(0) Call value = = = 13.6 1 + r 1 + .01 OR Step 1 Set up binomial tree and calculate the option values at expiration for each ending stock price. Step 2 Solve for the amount to invest in the stock and the amount to borrow in o
34、rder to replicate the option given its value in the up state and its value in the down state. Solve these equations simultaneously. Step 3 Use the values derived in Step 2 to solve for the value of the option at the beginning of the period. OR Step 1 120 (C=20) 100 80 (C=0) Step 2 20 = 120 x D 1.1 x
35、 B 0 = 80 x D 1.1 x B -1.1(20) - (-1.1)(0) -22 D = = = 0.5 120(-1.1) - 80(-1.1) -44 C = 80(.5) 1.1 x B 0 = 80(.5) 1.1 x B B = 40/1.1 - 36.4 Step 3 C = 50 - 36.4 = 13.6 1(b). The binomial option pricing model is a discrete version of the continuous time Black- Scholes option pricing model. As the num
36、ber of intervals in the binomial model approaches infinity, the option value derived from this model approaches the option value derived from the Black-Scholes model. The binomial model is more flexible than the Black-Scholes model because it does not require one to assume constant interest rates an
37、d constant variance throughout the horizon. These values can be changed at any of the nodes in the binomial tree. However, the binomial model is more cumbersome to use since accuracy requires that the tree include many nodes. 2.CFA Examination III (1992) 2(a). An individual put will hedge an amount
38、of the underlying stock index equal to the underlying value of the put, which in each case here is the respective stock index times $100. Also, due to the differing betas, the portfolio and the stock indexes are expected to produce gains or losses in proportion to their respective betas. Thus the nu
39、mber of puts required to hedge the portfolio must be adjusted for the betas of the respective stock indexes relative to the beta of the portfolio. The number and cost of protective puts could be calculated indirectly or directly. Indirect method: (port)(Vport ) = (GAC)(VGAC) = (opt)(Vopt) The indire
40、ct method is based on the fact that an overall portfolios beta is equal to the weighted average of its component betas. Since we are seeking a zero beta portfolio, the left-hand side of the equation can be set to zero. For the S other relevant factors in the decision are correlation and liquidity. W
41、hile the hedge calculated in Part A is intended to protect GACs portfolio from a decline, the portfolio does not replicate any of the indices. The hedge could be less than perfect if the price movement of GACs portfolio does not track the index movement. In this context, the index which has the high
42、est correlation with GACs portfolio (the S the NYSE transaction would clearly be least desirable. The S the S then decreases in the portfolio value will be offset exactly by increases in the put option value. As the relationship between the strike price and the index value changes, however, a trade-
43、off enters into the investment decision. If the strike price is below the current index value, then the puts are “out of the money.” These options will be less expensive than “at the money” or “in the money” puts; however they expose the portfolio to potential loss, with the difference between the i
44、ndex value and strike price being analogous to the deductible on an insurance policy. For example, if the S this also enables assessment of option premiums against implied risk levels. The model can also be used to calculate hedge ratios, the change in portfolio value for a $1 change in index price.
45、 In summary, option pricing models allow the portfolio manager to quantify the relative pricing of options to determine which option gives the most efficient portfolio protection per dollar of cost. 3.CFA Examination II (1998) 3(a). Critique of Belief Joel Franklins belief is incorrect. There are tw
46、o fundamental kinds of options: American style and European style. An American option permits the owner to exercise the option at any time before or at expiration. The owner of a European option may exercise it only at expiration. If an option is at expiration, it will have the same value whether it
47、 is American or European. The owner of an American option can treat the option as a European option simply by postponing the decision to exercise until expiration. Therefore, the American option cannot be worth less than the European option. However, the American option can be worth more. The Americ
48、an option will be worth more if circumstances make exercise of the option before its expiration desirable. So, it may have a higher premium. 3(b).European-Style Options Value The formula to calculate a call option using put-call parity is c = S + p Xe-rt where c =the price of a European call option at time t S =the price of the underlying stock at time t p =the price of a European put at time t X =the exercise price for the option t =time to expiration = one year Therefore, from the information given, Call option = $4.408 = $43 + 4.00 - 45.00e-.055 3(c). Ef