Analysis of the Dependence of Stock Risk Based on Copula Theory

The rapid development of the economy emphasizes the importance of financial risk. Financial risk analysis can help people understand finance more deeply and reduce the loss of profits. This paper is about the dependence on stocks, which is very important for analyzing the dependence structure of stock market and the portfolio risk of investment market. The experimental data are the daily closing price data of shares of Midea Group and Gree Electric. Copula theory is used to fit the daily return data of Gree Electric and Midea Group. By establishing the correlation structure model of the stock market, the daily return data of Gree Electric and Midea Group are better si-mulated.


Introduction
The rapid development of the economy emphasizes the importance of financial risk. Longin & Solnik (2001) proposed that testing the hypothesis that international equity market correlation increases in volatile times is a difficult exercise and misleading results have often been reported in the past because of a spurious relationship between correlation and volatility. As a result of the complex relationship between stock markets and within the market, the fluctuation of one risk is likely to cause the fluctuation of other risk factors. The efficiency of the Chinese wheat and soybean futures markets is studied by Wang & He (2005).
The exact definition of Copula function is given by Nelsen (2006). Zhang proposed that copula technique is a kind of comparatively new method of financial risk analysis, whose core is to connect the co-distribution of many random variances with their fringe distributions. Copula function is very important in the 2. Copula Function Nelsen (2006) proposed that Copula function is a join function that connects the joint distribution function That is, the function (1)

Properties of Binary Distributed Copula Function
• For any 1 2 1 , , • If U and V are independent and obey the uniform distribution on [0, 1], then ( ) , C u v uv = .

Gaussian Copula Function and t-Copula Function
Two-dimensional Gaussian Copula function: where ρ is a dependent parameter, 1 1 ρ − ≤ ≤ , Two-dimensional t-Copula function: where the linear correlation coefficient between variables is 1 1 ρ − ≤ ≤ , the degree of freedom is k.
Gaussian Copula function and t-Copula function have symmetrical tails, but Gaussian Copula function can't capture the asymmetric tail correlation between random variables. The t-Copula function has a thick tail, which is very sensitive to the changes of random variables distributed in dimensions, and can better capture the tail correlation changes.

Construction of Copula Model
The steps to build copula model are as follows: 1) According to the daily closing price data, the daily return rate of stock is calculated and its statistical characteristics are described.
2) The kernel density estimation method is used to construct the edge distribution, so as to fit the variation characteristics and distribution characteristics of random variables.
3) Catching the dependent structure between random variables by Gaussian Copula function and t-Copula function.

Sample Selection
This paper selected the daily closing price data of shares of Midea Group and Gree Electric from July 14, 2007 to October 17, 2019, to study the internal relationship between two stock markets. The data comes from Oriental Fortune net.
According to the daily closing price, the daily return of the stock is obtained, and its frequency histogram is given below. It can be seen from Figure 1 and Figure   2 that the distribution of the two-time series is relatively uniform, showing the characteristics of peak and thick tail, but the two-time series are not normal distribution.

Nonparametric Estimation
Kernel density estimation method is selected for nonparametric estimation. It is  defined as: If the random sample of a single variable x is 1 2 , , , n x x x  , the nonparametric density estimation method of the probability density distribution function ( ) f x of x is defined as: where ( ) K is kernel function. Table 1 gives the statistics of daily return, such as mean, variance, skewness and kurtosis. The two-time series are not normal distribution since their kurtosis is large than 3.
This paper chooses Gaussian function as the kernel function since the time series of stock return has the characteristics of peak and thick tail.
It can be seen from Figure 3 and Figure 4 that the empirical distribution function of the stock daily return of Midea Group and Gree Electric is basically consistent with the estimation of the nonparametric kernel distribution, and the nonparametric kernel density estimation can well estimate the overall distribution of the sample.

Model Evaluation
In order to verify the goodness of fit of binary Gaussian Copula function and the binary t-Copula function, this paper selected the method of squared Euclidean distance. The smaller the square Euclidean distance is, the better the fitting effect of Copula function is. The square Euclidean distance can be calculated by the following formula:

Conclusion
The main contribution of this paper is to select the daily closing price data of shares of Midea Group and Gree Electric as the research object, use copula function   to establish the model, and analyze the daily return of stock. The results show that the binary Gaussian Copula function and the binary t-Copula function can simulate the daily return data of the stock market very well. Copula function is very important in the analysis of market risk. Copula function can be used to better simulate financial data and decompose financial risks, so as to reduce the impact of financial risks. This paper only selects the data of two stocks in China, which are not very representative. In order to better study the role of Copula function in financial risk analysis, it is necessary to select more representative data.