Abstract
Recently, hybrid stochastic and local volatility models have become an industry standard for the pricing of derivatives and other problems in finance. In this study, we use a multiscale stochastic volatility model incorporated by the constant elasticity of variance to understand the price structure of continuous arithmetic average Asian options. The multiscale partial differential equation for the option price is approximated by a couple of single scale partial differential equations. In terms of the elasticity parameter governing the leverage effect, a correction to the stochastic volatility model is made for more efficient pricing and hedging of Asian options.
1. Introduction
Since the well-known work of Black and Scholes [1] on the classical vanilla European option, there has been concern about the pricing of more complicated exotic options. An exotic option is a derivative which has a payoff structure more complex than commonly traded vanilla options. They are usually traded in over-the-counter market or embedded in structured products. Also, the pricing of them tends to require more complex methods than the classical Black-Scholes approach.
This paper is concerned with one of the exotic options called an Asian option. This option is a path dependent option whose final payoff depends on the paths of its underlying asset. More precisely, the payoff is determined by the average value of underlying prices over some prescribed period of time. The name of “Asian” options is known to come from the fact that they were first priced in 1987 by David Spaughton and Mark Standish of Bankers Trust when they were working in Tokyo, Japan (cf. [2]). The main motivation of creating these options is that their averaging feature could reduce the risk of market manipulation of the underlying risky asset at maturity. Since Asian options reduce the volatility inherent in the option, the price of these options is usually lower than the price of classical European vanilla options. Note that there are two types of Asian options depending on the style of averaging: one is the arithmetic average Asian option and the other is the geometric average Asian option.
Since there is no general analytical formula for the price of Asian option, a variety of techniques have been developed to approximate the price of this option. Subsequently, there has been quite an amount of literature devoted to studying this option. For instance, Geman and Yor [3] computed the Laplace transform of the price of continuously sampled Asian options. However, there is a problem of slow convergence for low volatility or short time-to-maturity cases as indicated by Fu et al. [4]. Linetsky [5] derived a new integral formula for the price of Asian options but with the same convergence problem. Apart from this approximation technique, there are Monte Carlo simulation approach and partial differential equation (PDE) approach. Monte Carlo simulation methods typically require variance reduction techniques and also have to take into account the discretization error caused by discrete sampling. Refer to Kemna and Vorst [6] for a discussion of the pricing of Asian options with Monte Carlo methods. On the other hand, the PDE methods must deal with an extra state variable representing the running sum of the underlying process, which leads to an issue for reducing the dimension of the PDE. Refer to Ingersoll [7] and Rogers and Shi [8] and Vecer [9] for the PDE methods.
It is well known that the constant volatility assumption, on which a review of literature quoted above is based, for the underlying asset price is severely in contrast with many empirical studies which demonstrate the skew or smile effect of implied volatility, fat-tailed and asymmetric returns distributions, and the mean-reversion of volatility. Thus a number of alternative underlying models have been proposed. The constant elasticity of variance model by Cox [10], a stochastic volatility model by Heston [11] or Fouque et al. [12], and a Levy model by Carr et al. [13] are among those representative ones that can reproduce an empirically reasonable outcome.
So, it is desirable to study Asian options based on these alternative models. In fact, there are a number of recent studies along the lines of this type of extension. For instance, B. Peng and F. Peng [14] for the CEV model, Fouque and Han [15] for a stochastic volatility model, and Lemmens et al. [16] for a Levy type model are among those works extending the price for the Black-Scholes framework with constant volatility. As long as we understand, up to now, however, there has been no work for Asian options based on a hybrid stochastic and local volatility model. Recently, hybrid stochastic and local volatility models have become an industry standard for the pricing of derivatives and several financial institutions have incorporated those models into their systems [17]. Therefore, it is worth studying Asian options on the hybrid models.
This paper studies the pricing of an arithmetic Asian option under a hybrid stochastic and local volatility model which was introduced by Choi et al. [18], where the volatility is given by the product of a multiscale stochastic process and a power (the elasticity of variance) of the underlying's price. The hybrid nature of this volatility enables us to capture the leverage effect produced by the constant elasticity of variance (CEV) model as well as the smile effect of implied volatility, fat-tailed and asymmetric returns distributions, the tendency of the volatility process to revert towards a long-term mean at a certain rate, and a degree of correlation between the randomness of volatility and the randomness of underlying's price produced by a “pure” stochastic volatility (SV) model. This hybrid model is called the SVCEV model. So, this paper will generalize both [14, 15] into an approximation problem for the price of arithmetic average Asian options based on the SVCEV model.
This paper is structured as follows. In Section 2, we review the SVCEV model introduced by [18]. A multiscale partial differential equation for the price of an arithmetic average Asian option is obtained in Section 3. Section 4 is devoted to obtain an approximated option price under the condition of fast mean-reverting volatility. In Section 5, the effect of hybrid structure on the pricing of the Asian option is illustrated by a numerical study.
2. The SVCEV Model
In this section, we establish a partial differential equation for the price of Asian floating strike call option based on the SVCEV model.
As introduced by [18], the SVCEV model for the underlying's price is given by the stochastic differential equations (SDEs): where and are some constants, is greater than , and are positive constants, and and are correlated Brownian motions such that for some . If , then may fail to be a true martingale since may go infinite. Refer to [19]. Therefore, is assumed here. It is observed in most financial markets that there is a minus correlation or leverage effect between stock price and volatility impact. However, some commodity markets show the opposite effect (the inverse leverage effect). So, the model under the negative correlation condition can apply to many financial markets but has limits for some commodity markets. Also, generally speaking, the correlation may rely on time but it is assumed to be a constant for simplicity. In fact, in most real situations, it is taken to be such. We do not specify the function but it has to satisfy a growth condition to avoid the nonexistence of moments of . It is assumed in this paper that for some constants and .
From the to formula, the solution of the second equation in (1) is an ergodic process given by the Ornstein-Uhlenbeck process: and so , which has an invariant distribution given by . Later, we will use notation for the average with respect to this invariant distribution; that is, for arbitrary function .
Now, we take the process to be a fast mean-reverting process as in Fouque et al. [20]. This means that we take into account a fast time scale volatility factor as a major driving force for the volatility movement of the underlying asset. This assumption can be justified by an empirical analysis of high-frequency S&P 500 index data. The analysis confirms that volatility is fast mean-reverting when looked at over the time scale of a derivative contract although it reverts slowly to its mean in comparison to the tick-by-tick fluctuations of the index value. See [20] for details. Also, this assumption will provide us with analytic tractability for an approximation for the price of the Asian option as also shown in [15]. So, in terms of a parameter, say , which is taken as positive and small, we assume that where which implies that the long run magnitude of volatility fluctuations remains fixed as a constant. Then, under a risk-neutral probability measure , we have where and are two Brownian motions under the measure whose correlation is given by and denotes the combined market price of risk given by Here, is the market price of volatility risk, which is assumed to be a bounded function depending on .
3. A PDE for Option Price
In this section, we utilize the generalization of Vecer's dimension reduction technique given by Fouque and Han [15] to derive a two-space dimensional PDE representation for the option price.
In this paper, a payoff function for arithmetic average Asian options is given by where is a function satisfying for any real number . Note that, when , it becomes a payoff for fixed strike Asian options, whereas, when , it becomes a payoff for floating strike Asian options. So, the option price is defined by under a risk-neutral measure .
First, we would like to replicate the averaged process with a portfolio Here, is assumed to be a nonrandom function. By the self-financing strategy, we note that The assumption of the nonrandomness of yields Thus, using (11) and (12), one can obtain By integration of (13), we obtain which can be written as For technical reason, if we choose the trading strategy as and the initial portfolio price as then the final portfolio price becomes and so the general payoff function of arithmetic average Asian options given by (8) becomes . Refer to [9, 21] or [15] for details. In terms of the portfolio process , the price of an arithmetic average Asian option at time is given by
Next, we change the probability measure into the probability measure defined by
Then we are ready to obtain the option price as the solution of a partial differential equation.
Theorem 1. Let . Then the Asian option price at time can be expressed by under the measure , where If , then is given by the solution of the PDE: with the terminal condition
Proof. By the Itô formula, we obtain the following two results:
Then we have
where
which is a Brownian motion under the measure by the Girsanov theorem (cf. [22]).
From (8) and (18), we have
where
If , the Asian option price at time is given by
Then, by the Feynman-Kac formula (cf. [22]) and (25), one can obtain (22).
Once the solution is determined, from the result of Fouque and Han [15], the price is given by where is the solution of the PDE (22) at time .
4. Multiscale Analysis
In this section, we are interested in the solution of the multiscale PDE (22) in the asymptotic form . Then one can come up with single scale PDEs much easier to solve than the PDE (22) itself as follows.
Substituting the asymptotic form of into the PDE (22) yields with the terminal condition where
Note that is the infinitesimal generator of the Ornstein-Uhlenbeck process . Then we have a hierarchy of PDEs as follows: with the following terminal condition:
In the following two theorems, we obtain PDEs for the leading order term as well as the first correction term .
Theorem 2. Assume that the term does not grow as much as with respect to the variable for . Then the leading order term is independent of and satisfies the PDE problem: where
Proof. From the term in (34) and the fact that is the generator of , the growth condition of with respect to the variable leads to the fact that does not depend on . So, it is represented as without dependence on the variable . From this fact, equation becomes and so, by the same reason as for , also does not depend on the variable : Since is an operator in which every term contains the derivative with respect to the variable , becomes . By applying the Fredholm alternative (cf. Ramm [23]) to this Poisson equation with respect to the operator , we have ; that is, By adding the boundary condition (35) for to this PDE, we obtain the PDE problem (36) as desired.
Next, we obtain a PDE for the first correction term .
Theorem 3. Assume that the term does not grow as much as with respect to the variable for . Then the first order correction term is independent of and satisfies the PDE problem: where and are constants, respectively, given by Here, is the solution of
Proof. Since and , we obtain
Then, from the definition of , we obtain
for some function independent of .
By applying the Fredholm alternative to the Poisson equation , we have equation ; that is,
Combining (45) and (46), we obtain (41).
5. Approximate Option Price
From the results in Section 4, we formally obtain the first order approximation: where and are independent of the unobservable variable and they are given by the PDEs in Theorems 2 and 3, respectively. Note that solves a homogeneous equation with the nonzero final condition while solves a nonhomogeneous equation with the zero final condition. In terms of accuracy of the approximation (47), if the payoff function is smooth enough, it follows straightforwardly from [20] that the approximation is of order in the pointwise convergent sense. Otherwise, it requires a regularization of the payoff as done for European vanilla options in [24]. Since the relevant argument (with an exact form of the density function for ) is omitted in this paper, we limit ourselves to the case of regularized payoffs here.
If we define and by respectively, then, from (30), the option price has an approximation given by
In this section, we compute numerically the leading order price and the first correction term by using the finite difference method (the Crank-Nicolson method). The solution has the truncation error , where and .
Figure 1 shows and at time for three different values of (the elasticity parameter). The parameter values used in this figure are , , , , , , and . Note that the parameter is absorbed by and . We call the price the (approximate) SVCEV price for the arithmetic average Asian option. It can reduce to the price for the well-known models. In particular, the price with is the Black-Scholes price. The price with corresponds to the stochastic volatility model studied by Fouque and Han [15]. Let us call it the SV price. When the leverage effect () takes place, the SVCEV price becomes lower than the SV price. When the inverse leverage effect () takes place, the SVCEV price becomes higher than the SV price. Regardless of the value of , the first order stochastic volatility correction term is positive and has the maximum value near .
| (a) |
| (b) |
Figure 2 shows the correction term with respect to the group parameters and . This figure shows that the correction term decreases as the parameter increases whereas it increases as the increases. Figure 2 also shows that as the parameter becomes larger, the correction term becomes larger.
| (a) |
| (b) |
6. Conclusion
A frequent criticism of the stochastic volatility or local volatility models for path dependent options is that they do not produce deltas precise enough for hedging purposes. So, relevant industry experts recommend using a hybrid stochastic local volatility model of their own development for best pricing option products. See, for instance, [25]. In this paper, by transforming the path dependent problem for Asian options in the hybrid SVCEV model into a European vanilla style problem, we approximate the volatility dependent price by the nonvolatility dependent price. The approximate price provides not only a correction to the price of [15] for different values of the elasticity of variance parameter but also a correction to the CEV price based on the assumption of fast mean-reverting stochastic volatility. The elasticity of variance plays an important role in characterizing volatile markets as well as differentiating commodity markets from financial markets. Refer to [26]. The numerically solved Asian option prices show that both the leading order option price (the CEV price) and the corrected price (the approximate SVCEV price) increase as the elasticity of variance goes up. This result can provide more efficient risk hedging by choosing an appropriate elasticity parameter based on the observed market volatility or the chosen commodity.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The authors would like to thank the anonymous reviewers for their valuable comments and suggestions to improve the quality of the paper. The research was supported by the National Research Foundation of Korea NRF-2013R1A1A2A10006693.