Realized Volatility and Option Time Value Decay Patterns

Options have time value that decays with the passage of time. Whereas the Black-Schole model assumes constant volatility in the underlying, the realized volatility in the underlying can be nonconstant. Option time value decay has a pattern that is driven by market activity and market events, which matches the observed pattern in realized volatility. Introduction Option pricing remains a significant challenge in quantitative finance due to its uncertainty. There have been numerous efforts in academia and industry to develop models for modern-day markets. One of the parameters that remains uncertain is the time decay. Options are financial instruments that provide right but not obligation to purchase or sell an underlying asset before a certain expiry. The value of an option contract origins from the optionality factor that the investor can choose to opt in or out based on market information. This optionality is correlated with time to expiry, since there would be less uncertainty in the underlying, therefore less profitability to hold such optionality. However, the time value is not linearly correlated with time. The common practice is to use the Black-Scholes formula for pricing, which takes in parameters such as the underlying volatility and time to expiry. The model assumes constant volatility and in such, the time value can be seen as a function of time, holding other parameters constant. However, the real financial market has nonconstant volatility, and the time value of an option in the formula becomes a function of the product of time and variance, which this paper aims to build on. In fact, the change of volatility is related with the rate of price discovery in the market on the underlying. However, there is no simple way to represent such value. Since the effect of time decay has received little academic interest. However, in high frequency trading industry, where high precision is required, there are proprietary models developed by firms, but the details on them remain a secret. Some traders remove weekends from the price decay, believing that optionality only changes during trading hours. However, price discovery is effectively still ongoing in during non-trading hours, but is not necessarily reflected on the market. As exchanges created afterhours and pre-open trading, we gain another way to look into the price discovery process during non-regular trading times. First of all, the time value of an option represents the time left for the option until expiry, in which the option may fluctuate (Hull 2008). According to Hull, the time decay is usually measured by the rate of loss in value in the passage of a day. Theta is usually negative, as the option becomes less valuable as time passes due to decreased volatility and optionality. Although there is a prescribed formula to decay an option’s time value, there are restrictions in the industry that require modification to the approach. For example, Harwood talked about the different approaches to option time decay on weekends, where some traders decay by calendar day, while others by trading days (2015). Harwood mentioned that under most situations there is no decay when the market is closed, unless the option is close to expiration. He also mentioned one subtlety that he did not elaborate on, which is how market makers close out their positions prior to close on Friday. The role of market makers in financial markets is to provide bid and ask prices for assets (Hull 2009). They hold volumes of assets to quote on markets, and will provide liquidity when investors wish to buy or sell. The additional liquidity facilitates faster price discovery (Das, 2008). The price discovery process is the process of determining the price of an asset through interactions between buyers and sellers. There are researches on if the price discovery process in the options market affects the underlying market, but seemingly none in the opposite direction, at least publicly available (Muravyev et al., 2012, Holowczak et al., 2007). Therefore an important question is how price discovery and the amount of trading activity are related. This project mainly focuses on the discrepancy of trading activity across different times of the day. The fluctuation of trading activities across different trading hours is a widely observed and acknowledged phenomenon. The most common example is after hour trading. After hour trading refers buying and selling from an exchange outside of normal trading hours, which, although becomes more accessible to investors, consists of very small volume, at a mere 4% on Nasdaq, according to Barclay and Hendershott (2003). Moreover, according to Gerety and Mulherin, there exists an intraday pattern in trading volume in the equities market (1992). Admati and Pfleiderer studied the role of liquidity traders and informed traders in forming an intraday trading pattern on stock market, but only provided a model and still left questions unanswered. There has been observations that different amount of trading activities and announcements can cause shift in volatility, as indicated by Andersen and Bollerslev(1997), but there is no existing study on a detailed model on equity and crude oil. Previous studies often used volume as an indicator as trading activity, however, this study uses realized volatility in the underlying as the indicator, as it gives more indication to the lifting of quotes and less responsive to partly hitting quotes. Zhang stated that as high frequency trading becomes commonplace practice in recent years, it gives rise to increase volatility in the underlying market (2010). Zhang also concluded that HFT hindered price discovery, but only in the time scope of months out, whereas this study focuses on intraday. Simaan and Wu used trades that are in and out of the National Best Bid and Offer (NBBO) as a measurement of price discovery (2007). Compared to option time decay, there is much more groundwork done in the research on intraday volatility. Almgren proposed intraday volatility estimators that correct for factors that can arise in data collection in industry (2009). Zhou also proposed estimators that can reduce structural noise. Andersen and Bollerslev proposed a framework to integrate microstructure variables into standard volatility models for investigation (1997). This paper uses method proposed by Andersen as explained in the methodology section. In conclusion, this study will attempt to use existing measures in studying the dynamics of financial markets to apply to option time decay, a topic rarely touched in academia. Using the existing framework in volatility measurement in combination with the previous studies on price discovery, we can test our hypothesis on the data to see if our observation matches our assumption that realized volatility is a good measure of option time decay. Data The methodology proposed by this paper is independent of the underlying. In order to model the price discovery in the market, this paper chooses financial instruments that have large trading volumes and accessible underlying data, as well as an active options market. The two markets this paper will demonstrate the methodology on are the S&P 500 index and WTI crude oil markets. To calculate the realized volatility on the S&P 500 index, tick data from CBOE S&P 500 index (SPX), SPDR S&P 500 US equity ETF (SPY), and CBOE E-mini futures (ES) were collected from Bloomberg. ES futures that expire in February 2017 and March 2017 were used. For crude oil futures, CBOE WTI May 2017 futures CLK7 was used, collected from Bloomberg. To verify the model from observation, CBOE S&P 500 Volatility Index (VIX) data and observed implied at-the-money volatility for CL options were obtained from Bloomberg. Raw data contained both bid and ask on the minute tick. The following timestamps are central time. SPX and VIX data are tracked from 8:30 AM to 3:20 PM. SPY trades from 3:00 AM to 7:00 PM, with 8:30 AM to 3:30 PM being normal hours. ES and CL futures trades all day except between 4:15 PM 4:30 PM, and 5:00 PM to 6:00 PM. The tick data were interpolated to the five minute mark from 0:00 AM to 11:55 PM each day, if the tick is not present in raw data, previous observation is used. The time range SPX-related data is from January 3, 2017 to March 15, 2017. The time range for crude oil futures data is from January 20, 2017 to April 20, 2017. The week of March 6th was removed due to an abnormal, non-recurring market movement skewing the data. The data for crude oil options implied volatility is gathered for CLM7, CLJ7 and CLK7. The daily implied volatility is collected in weeks that are 1-3 months prior to expiry. Weeks with non-trading weekdays and non-Wednesday EPA announcements were removed.