Abstract
Option prices are characterized by wide bid-ask spreads, much wider than the spreads of their underlying assets. This note discusses the various attempts to rationalize and link the two markets’ spreads with each other and explains why their failures are mainly due to the inability to accommodate trading frictions in theoretical asset pricing models. It also presents in summary form the partial results from the stochastic dominance approach to option pricing under proportional transaction costs that provide bounds for the spreads for index call options.
Introduction: “Will the true equilibrium option price please stand up?”1
The statement that introduces the subject of this paper is taken from one of the most detailed early empirical option market studies involving all reported trades and quotes in the 30 most active CBOE option classes during the mid-1970s. The frustration of the study’s author, Mark Rubinstein, is understandable: he wanted to study simultaneous trades in the option and its underlying asset in order to verify whether the Black–Scholes–Merton (BSM, 1973 [2,22]) model was good enough for trading purposes. What he noticed was that during several time periods when the underlying asset had not moved at all, the call options had traded at two or three different price levels, corresponding to significantly different implied volatilities. As an example he cited a call option on an underlying equity value of $37.5 that had traded at three equidistant prices of 3.25, 3.375 and 3.5.
It is easy to see why these option trades were taking place at different prices for the same underlying value: they were all lying within the option bid-ask spread. The spread is probably the most important component of the costs of trading in the option market that include also brokerage fees and execution costs and are collectively known as transaction costs. In turn, these costs imply a range of indeterminacy of the “true” price of the options. In the example cited by Rubinstein this indeterminacy implied that the “true” trading price of the call option at observation time t, around noon on January 5, 1977, could be defined at best up to an error of 7.4% with respect to its midpoint. Its theoretical and empirical consequences for option market studies form the topic of this essay.
In the next section we provide some empirical evidence about the magnitude of the problem using data from the most liquid and actively traded options, those on the S&P 500 index. In Section 3 we discuss the (mostly failed) attempts to incorporate the existence of the bid-ask spread through the traditional arbitrage-equilibrium approaches introduced by BSM and followed by most option researchers. Section 4 reviews the (rather skimpy) alternative literature on transaction costs and the empirical studies that underlie it. Section 5 concludes and suggests avenues for future research.
The size and consequences of the option bid-ask spread
The 7.4% interval of option trading prices is a major understatement of actual conditions in option markets, given that the width of the quoted bid and ask prices is a function of the option’s maturity and degree of moneyness
The table shows the average difference of ask and bid prices of call and put options on the S&P 500 index as a percentage of the spread midpoint as recorded on every trading day at 2 PM, for two maturities in days and three different degrees of moneyness. ITM and OTM moneyness corresponds to
respectively for calls and
for puts, with
denoting the index value and K the strike price
The table shows the average difference of ask and bid prices of call and put options on the S&P 500 index as a percentage of the spread midpoint as recorded on every trading day at 2 PM, for two maturities in days and three different degrees of moneyness. ITM and OTM moneyness corresponds to
Observe how quickly the size of the spread increases for OTM options, even though this data refers to very near the money options. On October 2, 2008, at around noon, the spread was around 10% ATM for the November options, but rose to 20% for 6.4% OTM options. Further, these spreads show no sign of having decreased over time: in two different dates distant by about 21 years, between February 1992 and February 2013 when the index went from 411 to 1616, the spread for 28-day 2.5% OTM calls went from 14.3% to 23.3% and for the same 2.5% OTM 14-day calls from 25.6% to 41.9%. Observe also that these option spreads are far, far above the corresponding ones for the underlying asset: although the index itself is not traded, the exchange-traded SPDR fund tracks it very closely, and its observed bid-ask spread rarely exceeds 0.1%. Even if we double this amount it would be more than 100 times smaller than the one corresponding to the 2013 28-day OTM call options.
In the more than thirty years since the Rubinstein article there has been an explosion in theoretical and empirical option market studies whose complexity and level of mathematical sophistication were not even conceivable in those early years. Almost all the theoretical models of asset pricing have assumed a frictionless world, an assumption that yields elegant market equilibrium results, especially when markets are complete and asset dynamics can be represented by diffusion processes in continuous time.2
When markets are incomplete because there are rare events or volatility is stochastic no arbitrage was supplemented by market equilibrium considerations, in which a “representative” investor was assumed to exist, almost always of the constant relative risk aversion (CRRA) class. A variant of this approach is to assume similar asset dynamics in both the underlying asset and the option market and to extract the equilibrium conditions empirically from both markets. The frictionless assumption is very convenient because it offers a one-to-one link not only between option and underlying asset but also between options on the same underlying with different characteristics. From the observed option prices one can estimate the risk neutral distribution
Applying these elegant results empirically involves choosing an observed variable that represents the option price. Most studies have chosen the midpoint of the observed spread and assumed that the frictionless economy conditions such as put-call parity held. This creates problems, since applying parity to the observed midpoint put and call prices for every strike gives rise to other types of violations of the conventional no arbitrage relations, which must be removed by suitably “purging” the data since otherwise no option pricing model is conceivable. These adjustments are major: as a recent study points out,3 less than 1% of the observed S&P 500 option midpoints during 2006–2007 were arbitrage-free. Even the less drastic and more reasonable approach that seeks a vector of option prices for all strike prices in each cross section that is arbitrage-free and lies within the bid-ask spread yielded results that were less than fully satisfactory. The same study mentioned above shows that over 2006–2007 about 30% of the S&P 500 option cross section failed the test, implying that for these cases there was no arbitrage-free price lying within the observed bid and ask prices of both puts and calls. Similar results with a slightly different methodology were also found in another study that used a larger data base and more extensive tests.4 Last, there are persistent empirical findings within mainstream frictionless asset pricing models that are treated as anomalies within the models, such as the well-known “overpricing” of S&P 500 ATM and OTM index put (but not call) options, an implicit admission that put-call parity is violated, as is normal in the presence of the bid-ask spread.5 In the following sections we review the literature that has recognized this spread and attempted to deal with it.
In studying financial markets for any asset the determination of the spread is generally modeled by assuming that there are dealers or market makers who hold diametrically opposite positions to those of investors: the quoted ask (bid) price at which investors can buy (sell) the asset is the lowest (highest) price at which market makers are willing to sell (buy) the asset. In the case of S&P 500 options market makers hold positions on several options in a given cross section as well as an inventory of underlying asset such as tracking fund or futures contract, since this is the cash equivalent that will be delivered upon exercise or expiration of the options. Since the inventory size cannot be determined in the absence of a pricing model that recognizes frictions, there is a chicken and egg problem in trading derivative assets: market making cannot be done efficiently without a good pricing model, and such a model cannot be formulated without taking into account the bid-ask spread.
Such modeling in a no arbitrage context with diffusion asset dynamics was introduced by Leland [19] and subsequently extended in a binomial model by Merton [23] and Boyle and Vorst [5]. These articles applied variants of the dynamic option replication policy of frictionless no arbitrage to a universe that admits proportional transaction costs in trading the underlying asset. Market makers hedge their positions perfectly by constructing portfolios using the underlying and the riskless asset whose final payoffs at maturity equals those of the long and short options respectively, and deliver them to the option holders at expiration. As Merton [23] pointed out, in a continuous time model this would imply a continuous rebalancing of the portfolio to reflect the new underlying price and incur an infinite volume of trade over the lifetime of the option. In a binomial model it would imply restructuring the portfolio at every node of the binomial tree, thus making the bid-ask spread a function of the time subdivisions till option expiration.
The consequences of such restructuring are more clearly evident in the binomial model. As Boyle and Vorst [5] showed, the portfolio replicating the long option tends to a BSM-type model as the time partition
Stochastic dominance bounds: Theory and empirical applications
Since the no arbitrage approach fails in the presence of transaction costs, it was perhaps natural to turn to joint equilibrium in the underlying and option markets to search for an alternative. The first step in market equilibrium models is portfolio selection in the presence of transaction costs, for which the literature is rather sparse. To our knowledge asset allocation models involving one risky and one riskless asset are the only ones for which usable results are available. Their main features are reviewed briefly before linking them to the option market.
Asset allocation under proportional transaction costs
Consider a class of traders who invests only in a risky asset and a riskless bond, a condition that will be assumed throughout this section. Each trader makes sequential investment decisions in the primary assets at the discrete trading dates
The trader enters the market at date t with dollar holdings
This problem was examined in a seminal paper by Constantinides [8], which derived the properties of the value function
The key issue in attempting to solve this problem for a specific utility function, even for the simple CRRA class, is the assumption about the asset dynamics governing the risky asset return
Option bounds under transaction costs
The derivation of the European option bounds combines the investor asset allocation problem under transaction costs with the frictionless stochastic dominance (SD) methodology introduced into option pricing by several studies during the 1980s.12 In that approach the trader considers whether there exist zero net cost portfolios involving long or short positions in a single option, possibly combined with the underlying asset and the riskless bond that can be shown to increase her expected utility without depending on its parameters, the initial wealth or its composition. Alternatively, it adopts the pricing kernel approach to pricing assets and uses linear programming (LP) to determine the kernel that maximizes or minimizes the equilibrium price of a single option given that it depends only on the risky asset’s return distribution.13 For options that are convex with respect to their underlying price both methods derive the same upper and lower option bounds for call and put options, American and European, for index options. These can be extended to equity options if one assumes a linear structure in equity returns similar to the capital asset pricing model (CAPM). In the case of diffusion asset dynamics both bounds have been shown to converge to the BSM price for both index and equity options.14
The extension of SD in the presence of proportional transaction costs is not trivial and yields results only for some types of options. Specifically, we have tight upper bounds for both European and American index and index futures call options, and lower bounds for the same call options, as well as for European and American index and index futures put options, while the upper bounds for put options are less useful and depend on the time partition.15
The key property is the concavity of
Empirical tests
To our knowledge no empirical testing of the efficiency of the option market making function exists, although the verification is easy and depends on the assumed distribution of the underlying asset. On the other hand, in verifying the observed price
To test these violations out-of-sample the zero-net-cost portfolios used in deriving the bounds were evaluated at each data point, and their realized payoffs at option expiration were added to the proceeds of investing in a unit of the index, forming a time series of the option trader (OT) returns whose initial cost was equal to that of one unit of the index. After suitable standardization this total OT payoff series was compared to the series of realized returns of one index unit, the index trader (IT) series, in a test of stochastic dominance of one time series over another. The chosen test was by Davidson [16] and Davidson and Duclos [17], in which the null was that of non-dominance of the OT over the IT series, and rejection of the null implies the OT payoffs series dominates the corresponding IT series. The test statistic was evaluated over the entire time series, including the dates where no bound violating option existed, and rejected decisively the non-dominance null. In terms of average OT excess returns, these varied from 0.31% to 0.66% annually, which were risk-adjusted by virtue of the SD relationship; note that the ex-dividend average index return, the IT return, was only 4%.
We elaborate further on the methodology underlying the out-of-sample results arising out of the violations of the call upper bound (5). Whenever a call bid price exceeded (5) the portfolio to exploit the violation adds to the index a short option, yielding a combined OT payoff at option expiration equal to
These results were generalized and extended more recently in an ongoing study of short term (28-, 14- and 7-day) S&P 500 index options. The shifting of positive payoff from high to low index return range is being applied to the OT portfolios in the new study, with the difference that the portfolio composition is not predetermined but found numerically. The algorithm searches among the universe of traded options for zero net cost portfolios of calls and puts, long or short, written or purchased at the appropriate bid or ask prices, such that the payoff at expiration lies above
Conclusions and implications for further research
We started with a reminder of a problem that was pinpointed more than 30 years ago, but that is still present in the overwhelming majority of empirical option pricing studies: the inability to define the appropriate price of an option given the wide spreads at which options are traded in the option market. This problem is more acute for OTM options and does not show any signs of going away with time. Further, the OTM options have, if anything, increased in importance in the recent asset pricing literature, since the development of volatility trading relies overwhelmingly on them.19 We pointed out that neither the observed trading price nor the spread midpoint are satisfactory proxies for this unobservable variable. We also noted that the incorporation of transaction costs into the no arbitrage methodology without changing its basic replication assumptions leads nowhere and can be at best considered as an approximation whose accuracy and cost are a priori unknown.
We then focused on the sparse literature that recognizes frictions in option trading without falling into the continuous trading trap of no arbitrage. This literature brings utility functions explicitly in the modeling of market maker behavior without tying itself to a specific investor utility function, especially its dependence on the (unobservable) risk aversion. Although it has produced comparatively few theoretical results, these make possible the (partial) empirical verification of the efficiency of market making in the index call option markets and the extension of the no arbitrage market efficiency condition by incorporating the “no stochastic dominance” condition into the index option markets. It also introduced rigorous statistical methods of the ex post (out of sample) verification of option market efficiency, by adapting econometric tests developed for different purposes to options violating in-sample efficiency.
How can this empirical literature proceed, beyond verifying efficiency, to option pricing tests? A possible approach would be to proceed beyond the SD bounds, towards a formal derivation of a market maker’s reservation purchase and write prices based on specific utility functions, such that these reservation prices will leave the market maker’s initial utility level unchanged. Although closed form expressions are probably beyond reach, even with a CRRA utility function and jump diffusion asset dynamics, a numerical approach is probably feasible and will at the very minimum allow the determination of the limits on risk aversion implied by the bid-ask spread, as well as on the risk neutral distributions consistent with that spread. More complex asset dynamics need extension of the SD relations to bi-dimensional utility functions.
Footnotes
Acknowledgements
I wish to thank George Constantinides and Michal Czerwonko for their helpful comments. I am the sole responsible for any remaining errors.
This approach is also known as super-replication. See Bensaid et al. [1] and Perrakis and Lefoll [
].
The results extend routinely to the case that consumption occurs at each trading date and utility is defined over consumption at each of the trading dates and over the net worth at the terminal date.
If utility is defined only for non-negative net worth, then the decision variable is constrained to be a member of a convex set that ensures the non-negativity of the net worth. The option bounds apply to this case as well.
See Liu and Lowenstein [21] for a continuous time solution under special conditions and Czerwonko and Perrakis [
] for a numerical solution under a more general formulation.
This assumption yields a kernel that is monotone decreasing in the risky asset return.
See Constantinides and Perrakis [13,
].
Unlike the frictionless case, SD cannot be applied to equity options when the CAPM holds since, unlike market makers, individual investors cannot be assumed to hold only the underlying and the riskless asset.
