1. Introduction
Dependence modeling among financial assets has been an important topic of research in quantitative risk management. For daily returns, there are many papers on the use of copulas to model dependence structures across different financial assets, such as
Dißmann et al. (
2013) and
Krupskii and Joe (
2020), among many others. However, for intraday financial risks, there has not yet been much work conducted. The recent market crash on 16 March 2020 tells us that intraday risks of the financial market can be as large as a loss of
in the major stock market index in a single day. With the growing availability of high-frequency financial data and the massive popularity of algorithm trading, it becomes more and more relevant to study intraday risks of the financial market. In particular, little is known about the intraday tail dependence patterns of financial assets and these are more relevant to better understand the intraday market operation and tail risks.
There has been a research trend on dependence modeling for high-frequency financial data. For example,
Koopman et al. (
2018) used copulas with time-varying parameters to show that in the US stock markets, the dependence starts low but gradually increases throughout the day. The copula was used by
Carvalho et al. (
2021) to investigate the dependence of the 15 min returns of the Brazilian stock market.
Buccheri et al. (
2021) investigated intraday dependence patterns in asynchronous high-frequency financial data.
Kim and Hwang (
2021) studied directional dependence of intraday volatility via the copulas.
Weiß and Supper (
2013) utilized vine copulas to find tail dependence between intraday returns and bid–ask spreads of Nasdaq stocks.
To our knowledge, there is lack of research work in the literature about tail dependence patterns of high-frequency financial returns, how they change over time between different stock categories and in different market regimes, and what factors contribute to such changes. From what we have learned in this study, intraday dependence patterns can be quite different than those in daily returns and they are worthy of further study.
Copula models such as Student-
t, Gumbel, Clayton, and BB series copulas such as BB1, BB4, and BB7 copulas studied in
Joe (
1997) are commonly used to model dependence among returns of financial assets, as those copulas can capture tail dependence in financial markets. For example,
Carvalho et al. (
2021) used the BB7 copula to model asymmetric tail dependence between financial returns. However, the commonly used copula models are not flexible enough in the tails to serve our purpose of modeling tail dependence, especially for conducting tail comparisons. The Student-
t copula, for example, is only symmetric between the upper and lower tails and can only handle relatively stronger tail dependence; the other commonly used copulas, such as the BB series copulas, cannot be symmetric between the upper and lower tails and a direct comparison between the upper and lower tails using such copulas is unfeasible.
To capture various intraday tail dependence patterns, one needs not only a model to capture the overall dependence but also a model that is tailored to be flexible enough to model the dynamic lower and upper tails and make comparisons between the tails without too much distraction. The full-range tail dependence copulas studied in
Hua (
2017) and
Su and Hua (
2017) are ideal candidate models for this study, due to their flexible upper and lower tails to capture various tail dependence patterns. In particular, we use the PPPP copula studied in
Su and Hua (
2017) to model the dependence between standardized residuals of high-frequency log returns of two financial assets, with each univariate margin being modeled by a multiple components GARCH model to account for intraday seasonality, volatility, etc. The two dependence parameters that are used to respectively control the lower and upper tails of the PPPP copula are linked to a generalized additive model with natural cubic splines being utilized to explain the nonlinear effects arising from the time and the Fama–French five factors. To quantify the strength of tail dependence for comparisons, we propose a model-based
unified tail dependence measure based on the PPPP copula. The unified measure ranges from 0 to 1 and can quantify the full range of positive tail dependence, covering both asymptotic dependence and asymptotic independence cases.
Through the regression analyses mentioned above, we have observed that, for QQQ and SPY, which are ETFs that track the Nasdaq-100 and S&P 500 indexes, respectively, the unified upper and lower tail dependence increases over time and tends to peak toward 4 p.m. in New York time, which is the end of the regular trading hours of the US stock market, and the tail dependence often plateaus or drops slightly during noon; this phenomenon also exists between AAPL and MSFT, the two main growth stocks. For the value stocks, we studied the pair of two major banks, JPM and BAC, for which the tail dependence can be higher after the market opens and before the market closes, with the noon trading session having the lowest unified tail dependence measures.
Market regimes also affect the overall dependence of the tail and the degree of asymmetry between the dependence of the lower and upper tails. It is commonly believed that for the returns of financial assets, the lower tail dependence is stronger than the upper tail dependence, and the lower tail dependence is more relevant when the stock market crashes due to some spillover or contagious effects (
Kato et al. 2022;
Rodriguez 2007). On the contrary, we find that high-frequency returns tend to have higher upper tail dependence than lower tail dependence, especially during a market sell-off, when both the upper and lower tail dependence increases, and furthermore, the lower and upper tail dependence patterns become a lot more different, much more than those during a market rally.
Among the Fama–French five factors (
Fama and French 2015), the factor of excess return on the market, which is the difference between value-weighted market returns and a risk-free return rate, affects the tail dependence patterns the most. Both the lower and upper tail dependence reaches the minimum when there is a moderate positive excess return. Some other factors such as “Small Minus Big” and “Conservative Minus Aggressive” also affect tail dependence significantly, with the exception that the “Robust Minus Weak” factor was not significant after the other four factors were controlled in our analyses.
We believe that our work is the first one to use a full-range tail dependence copula to study the tail dependence patterns of short-term financial asset returns. The proposed method of using a full-range tail dependence copula with regression on both the upper and lower tail dependence parameters provides an effective way of modeling tail dependence and their explanatory variables. The model-based unified tail dependence measure overcomes the difficulty of comparing tail dependence with different quantities in different situations. With the help of the proposed model, we have obtained some new empirical findings on tail dependence patterns for high-frequency financial data.
In what follows, we discuss the details with the following structures:
Section 2 explains the details of the regression models and covers the notion of full-range tail dependence copulas, the unified tail dependence measures, etc.
Section 3 presents an empirical study for some US stocks and ETFs that track the major stock indexes, and the main findings on intraday tail dependence patterns will be discussed in detail. Finally, we will conclude the paper in
Section 4.
2. Regression Models for Intraday Tail Dependence
In
Section 2.1, we will briefly describe how to use the copula to model the dependence between several time series data. The concept of the
full-range tail dependence copula and its properties will be introduced in
Section 2.2. To directly compare the upper and lower tail dependence, we propose using a model-based tail dependence measure referred to as the
unified tail dependence measure, which is discussed in
Section 2.3.
Section 2.4 introduces a GARCH model of multiple components and the model is used to account for intraday features of returns, such as seasonality and dynamic volatilities.
Section 2.5 discusses the generalized additive models for the regression analysis of dependence parameters.
To facilitate the reading, we provide in
Figure 1 a flow chart of the proposed models and their associated sections.
2.1. Copula for Dependence of Multiple Time Series
Sklar’s theorem (
Sklar 1959) tells us that for two continuous random variables
X and
Y with the joint CDF
F and the marginal CDFs
and
, respectively, the copula function
characterizes the dependence structure between
X and
Y. We refer to
Joe (
1997) for basic concepts about copulas and various parametric copula functions.
Copula functions, unlike many other dependence measures in quantitative finance, such as covariances and correlations, have the advantage of capturing non-linear dependence structures, regardless of how flexible the dependence patterns are; see, for example,
Patton (
2009). It is especially useful to capture the dependence between financial asset returns, which is generally higher during a market sell-off or rally; see
De Luca and Zuccolotto (
2017). Copula functions can be utilized to capture dependence in the upper and lower tails for such stronger dependence structures that can be caused by greed and fear of the market participants.
Copula functions are frequently used to model residuals that arise from each marginal time series, which are usually modeled individually by their appropriate models. In
Patton (
2012), for example, log returns of each financial asset are represented using commonly known time series models to account for mean and variance processes, and copula functions are utilized to model residual dependence. The dependence structures that are modeled by copulas are actually the dependence structures of some “deviance” from their expected values after accounting for the past information on expectations and volatilities, and the dependence suggested by the copula fitted based on the standardized residuals reflects how new information affects both the marginal time series. We use the copula to model such dependence structures between time series, but it should not be simply understood as the dependence between the observed time series data, and it only explains how marginal time series depend on each other after new information arrives. For the intraday financial data, we also use the copula to model their standardized residuals. To account for dynamic intraday volatility patterns, we employ multiple component GARCH models, and ARMA models are applied for the mean processes of log returns. Then, standardized residuals are utilized to fit the copulas; see
Section 2.4 for details.
2.2. Full-Range Tail Dependence Copula
For the copula C, if , then is referred to as the lower tail dependence parameter of C. Let be the survival copula of C. If , then is referred to as the upper tail dependence parameter for C. In the literature, the tail dependence parameters are also known as tail dependence coefficients and they can be used to describe the degree of dependence in either the upper or lower tails. If we are concerned about the lower tail, which corresponds to the dependence of high-quantile losses between financial assets, and when u is a small positive number very close to 0, then we can approximate the probability by , where and are random variables and and are the corresponding quantile functions. In practice, parametric copulas are frequently used to fit real data, and the estimated values of and can be used to assess risk and profit dependence, respectively.
The following patterns frequently appear in the dependence between financial asset returns: (1) The upper and lower tails usually have different dependence patterns. (2) In both the upper and lower tails, dependence patterns are usually dynamic over time. Commonly used copulas have some limitations that make them unsuitable for modeling and comparing tail dependence among financial asset returns. Student-t copulas, for example, are widely used in the literature to quantify the dependence among stock returns due to their ability to model tail dependence. However, unlike full-range tail dependence copulas that will be discussed in further detail in what follows, Student-t copulas are always symmetric between their upper and lower tails; tail dependence exists not only in the upper and lower tails for capturing extreme positive or negative dependence, but also between extreme positive and negative returns. It is unlikely that we will have an extreme dependence on both returns of the same sign and returns of the opposite signs at the same time. Other commonly used copulas, such as the Gumbel, Clayton, and BB series copulas, can only model asymmetric dependence between the upper and lower tails.
When the dependence on the tails is not strong enough,
or
can be 0. Then, one can use
as
to model the tail dependence pattern, for which
is a slowly varying function, and the parameter
is called the
tail order, which is then used to describe the dependence strength in the tail; see
Hua and Joe (
2011) for details of tail orders. We only consider non-negative tail dependence, so the range of interested values of the tail order is
.
When the lower tail dependence parameter , the lower tail order can provide further information on the dependence strength of the lower tail. However, all the most popular copula models can only model one scenario: or , but not both. To compare the strength of dependence in the upper and lower tails of intraday returns, we need a copula that has the following two critical features:
Being able to capture the full-range tail dependence (i.e., the set of values of
is dense in
) in both the upper and lower tails. We refer to Definition 2.1 of
Su and Hua (
2017) for a formal definition of full-range tail dependence.
Being able to model both reflection symmetry and reflection asymmetry between the upper and lower tails. That is, the model can handle both cases: for any and with a positive probability for some . Note that the concepts of “reflection symmetry” and “permutation symmetry” are different and the latter means that for any . We only consider copulas that have permutation symmetry that are mostly observed for dependence among returns of financial assets.
Constructing parametric copula families that have the above two common properties while also being computationally implementable has been a difficult task. So far, there are only two full-range tail dependence copulas: the GGEE and the PPPP copulas. Both are induced by
, which can be constructed by mixing four random variables of the following form:
where
’s and
’s are all independent. Since
and
share the same
and have independent
’s, the dependence between
and
becomes an interpolation of comonotonicty, the most positive dependence structure from the shared component
, and the independence structure. The names of the two copulas are based on what
’s and
’s are used: the PPPP copula is induced by such a mixture model of four Pareto random variables, and the GGEE copula is induced by two gamma and two exponential random variables. The GGEE copula was created in
Hua (
2017) and satisfies both the features 1 and 2 listed above. However, the current implementation in the R package
CopulaOne1 is quite slow in terms of computation speed. Later, the PPPP copula family was developed in
Su and Hua (
2017), which not only has the above features 1 and 2, but is also much faster computationally and is suitable for dealing with heavy computational tasks such as modeling dependence among high-frequency returns of financial assets.
The PPPP copula has four parameters
and the parameters enter the copula model as the shape parameters of the four Pareto random variables in the above stochastic representation. Since the shape parameter determines the tail heaviness of the Pareto random variable, an interaction between different levels of tail heaviness and the aforementioned interpolation between comonotonicity and independence leads to the flexible tail dependence structures. We refer to Assumption III of
Su and Hua (
2017) for mathematical details of the mixture model used to construct the PPPP copula. The normalized contour plots in
Figure 2 of the PPPP copula have clearly demonstrated how flexible it can be. As a result, we will use the PPPP copula to model dependence patterns among financial asset returns.
The following summarizes some advantages of using full-range tail dependence copulas to model dependence patterns for financial asset returns: (1) It can capture dynamic dependence patterns in both the upper and lower tails. (2) It models the upper and lower tails separately with the corresponding parameter for the upper and lower tails, respectively, so that the degrees of dependence in the upper and lower tails can be modeled separately and compared directly without inherited distraction from the model itself. (3) It can account for both reflection symmetry and asymmetry, allowing the difference between the upper and lower tails to be driven solely by the data rather than the model constraints. (4) It can capture the full range of tail dependence, from tail independence to the strongest positive tail dependence. (5) It can capture dynamic dependence with a single copula, eliminating the need for copula selection.
2.3. Unified Tail Dependence Measures
Tail dependence becomes very weak and close to independence as tail order
approaches 2; see
Hua and Joe (
2011) for reference. When comparing values of tail orders close to 2, comparisons become less interesting and less informative. As a result, we can constrain the range of tail orders to the most interesting range
by reparameterizing the parameters of the PPPP copula as follows:
and
. Then,
and
. We can achieve the ideal range of
according to Proposition 4.10 of
Su and Hua (
2017). It is especially useful when both the upper and lower tails have a very weak positive dependence and the full-range tail dependence copula is able to make comparing such weak positive dependence possible and informative.
We still have to deal with another issue after limiting the range of tail orders to . That is, we use the tail dependence parameters and to quantify the degree of tail dependence when the tail order . If , however, , and we must use and to measure the degree of tail dependence. With both and , comparisons between the cases of and become inconvenient; therefore, we need a unified quantity to allow such direct comparisons.
To address this issue, we propose to employ
where
and
are the corresponding parameters of the PPPP copula in order to quantify the strength of dependence in the lower and upper tails, respectively. In what follows, we refer to
and
as the
unified tail dependence measure of the PPPP copula in the lower and upper tails, respectively.
The fully parameterized PPPP copula has four parameters: , and b, and a comparison between the upper and lower tails is more sensible if because a and b affect the strength of tail dependence as well and we do not want different values of a and b to distort the comparison between the upper and lower tails. We set in the empirical study to focus on uncovering and comparing different tail dependence patterns, but not to aim to achieve a better-fitted model with potentially different values of a and b; letting is just for convenience and serves our purpose well enough. On the one hand, the unified tail dependence measures of and change continuously in the two cases: and , and, on the other hand, the strength of dependence in the tails increases in the values of and for the lower and upper tails, respectively, regardless of whether or . When , we have the usual lower [upper] tail dependence with , and when , we have intermediate lower [upper] tail dependence cases with . As a result, and are excellent metrics for capturing the degrees of dependence in the tails.
2.4. Multiple Components GARCH for Intraday Volatility
It is well known that temporal dependence exists in a single time series of financial prices or returns. To use copulas to assess the dependence patterns among different time series data, one needs to filter each time series data first, and then the copula is used to model residuals that are better than the original data to satisfy the identical and independent assumption required for fitting a copula.
As a result, we must first filter each time series data using appropriate models. Furthermore, when compared to volatility across days, intraday volatility of log returns has distinct characteristics, such as the fact that volatility is typically higher right after the market opens and before the market closes, while volatility is typically lower during lunchtime in the US market. We need a model that can account for both the seasonality of volatility within each trading day and the heterogeneity of volatility across trading days. In the following, we model log returns of each financial asset using a multiple components GARCH model.
Denote intraday log returns as , where is the day index with the total number of days being T and i is the index for time bars. Regular trading hours on US stock exchanges are from 9:30 a.m. to 4:00 p.m., New York time, totaling 6.5 regular trading hours. We use 30 s time bars, so and 780 is the number of 30 s time bars in the regular trading hours of the market. We only keep the time bars where both data are available. Since we only consider stocks and ETFs that have high liquidity, missing values are not an issue in our study.
The multiple components GARCH has the following specification:
where
is the stochastic component for the intraday volatility so that the innovation
can be assumed as a standard distribution such as the standardized Student-
t distribution;
is the volatility for the specific day
t and it can be modeled separately using some other models for daily volatility and then inserted into the model; and
represents the intraday seasonality for the
ith time bar and it describes the average intraday seasonality effect over the days that we are modeling. We refer to
Engle (
2002) and
Galanos (
2023) for details of the standard GARCH model of multiple components and its implementation in the R package
rugarch (
Galanos 2023). Then, we use the PPPP copula to model the standardized residual
’s for different financial assets. For each stock, the standardized residuals for day
t are converted into uniform scores
’s by their corresponding ranks within the day:
, where
is the total number of observations on day
t. When we need to study a particular trading session, such as the 90 min period before the market closes, we retrieve the corresponding uniform scores directly from all the uniform scores obtained for the entire day. Note that uniform scores are obtained on the basis of their daily ranks, even if we only use the data for a specific period within the day.
2.5. Regression Models with Full-Range Tail Dependence Copulas
To study how factors such as time of day and the Fama–French five factors affect intraday tail dependence, we perform regression analyses using the uniform scores of the standardized residuals from the aforementioned intraday marginal models as response variables, and the explanatory variables are linked via natural cubic splines to the upper and lower tail dependence parameters, respectively. Given observations
of the explanatory variables, we let the
enter the model via natural cubic splines as follows for both the upper and lower tail dependence parameters:
where
are continuous explanatory variables with
as the natural cubic splines for them and
as the categorical explanatory variables. In this study, we only consider continuous explanatory variables, but categorical variables can also be used in the model. The parameters to be estimated are
.
A maximum likelihood method can be used to obtain the estimates of these parameters, and the variance–covariance matrix of the estimated parameters can be derived from the approximated Hessian matrix. To assess the variability of the estimates, we can simulate regression parameters based on the estimates and the approximated variance–covariance matrix many times. Each time, we plot a line based on the simulated parameters (see the gray lines in Figures 5 and 7), and these simulated lines show the variability of the fitted regression lines.
4. Concluding Remarks
By modeling the evolving intraday tail dependence patterns, it is possible to discover some common but hidden dependence patterns in the financial market. We recommend using the PPPP copula, a full-range tail dependence copula, to model standardized residuals for log return processes to capture dynamic tail dependence patterns between financial assets. To account for the unique characteristics of intraday volatility, a standard GARCH of multiple components model is required. To compare tail dependence at various levels, a model-based unified tail dependence measure is suggested. Instead of using different measures in different situations, this metric enables direct comparisons of tail dependence strength in various situations. Through the empirical study, we have uncovered several interesting intraday tail dependence patterns for popular US stocks and ETFs over time via explanatory variables such as the Fama–French five factors. We have attempted to explain the reasons for these dependence patterns, but more empirical research is required to justify them. The proposed method and the unified tail dependence measures can also be applied to other financial assets.
The study of intraday tail dependence in financial markets provides more aspects of the market to both investors and policymakers. It can help them better understand the sentiments of the intraday market and the behavior of market participants. It offers insights into extreme events, which are often the most challenging and important for risk assessment and policy decisions. The current study is limited to the year 2020 and the selected ticker symbols. However, the proposed method can be applied to many other research questions. For example, a significant asymmetry between the upper and lower tail dependence could be helpful in detecting manipulative market behavior and the footprints of influential market players. Future studies can be carried out to examine a large number of stocks over multiple years. Such a study would be helpful to better understand the dynamic tail dependence patterns in different market environments and for different categories of financial assets and thus help prevent and manage large intraday financial risks.