Analysis and forecasting of crude oil price based on the variable selection-LSTM integrated model

In recent years, the crude oil market has entered a new period of development and the core influence factors of crude oil have also been a change. Thus, we develop a new research framework for core influence factors selection and forecasting. Firstly, this paper assesses and selects core influence factors with the elastic-net regularized generalized linear Model (GLMNET), spike-slab lasso method, and Bayesian model average (BMA). Secondly, the new machine learning method long short-term Memory Network (LSTM) is developed for crude oil price forecasting. Then six different forecasting techniques, random walk (RW), autoregressive integrated moving average models (ARMA), elman neural Networks (ENN), ELM Neural Networks (EL), walvet neural networks (WNN) and generalized regression neural network Models (GRNN) were used to forecast the price. Finally, we compare and analyze the different results with root mean squared error (RMSE), mean absolute percentage error (MAPE), directional symmetry (DS). Our empirical results show that the variable selection-LSTM method outperforms the benchmark methods in both level and directional forecasting accuracy.


Introduction
Since 2014, the international crude oil price has experienced the most significant volatility since the 2018 financial crisis. The oil market has taken on new features that affect the development of the global economy, national strategic security, and investor sentiment significantly. Especially as the primary alternative energy resources, the US tight oil production has been significant macroeconomic effects on the oil price (Kilian, 2017). In 2014, US shale oil producers plundered market share, leading to a change in global crude oil supply and demand balance. According to EIA, US shale oil production increased from 4.96 million barrels per day in 2017 to 5.59 million barrels per day in 2022. In addition, there are geopolitical events, trade frictions, and OPEC's agreement have occurred in recent years, causing the volatility of oil price. The internal and external environment of the oil market is changing, and the influencing factors have become diverse and complex. As the factors affecting international oil prices become more and more complex, it becomes difficult to capture practical factors and predict oil prices. Many past kinds of literature about crude oil price forecasting show that the forecasting results are sensitive to the modeling sample data frequency and data interval selection (Yu et al., 2019;Yu et al., 2008a;Zhang et al., 2015). As a strategic resource, crude oil plays a vital role in national energy security.
Meanwhile, with the financial properties of crude oil strengthened gradually, the volatility of crude oil prices is bound to affect oil companies' earnings and investor behavior. Therefore, systematic analysis of the characteristics of complex international oil markets and accurate capture of the new trend in international oil prices are critical. However, as the linkage between the markets, the uncertainty of the world economy and energy, the influence factors of oil price have become complex. It is difficult to point out which factors have the dominant effect on the oil price. If all possible oil price factors are added into the existing forecast model, it may lead to over-fitting problems, which will affect the forecast results. How to forecast crude oil prices in a new and effective method is one problem that academics and practitioners are very concerned about all the time. It can provide reference and theoretical support for the formulation of national energy security strategy and enterprise avoidance of market risks. To better analyze the changing trend of the crude oil market, it is necessary to determine the main factors affecting the price, determine the impact of each factor on price, and establish a forecasting model finally.
The research on the prediction of international oil price has always been a hot topic. A large number of papers with theoretical and practical application value have appeared. We make a simple review from two aspects of influencing factors and forecasting methods as follow:

Influencing factor
Most of the research has divided the influence factors of crude oil price into supply and demand, finance factor, technology (Hamilton, 2009a;Kilian & Murphy, 2014;Zhang et al., 2017;Wang et al., 2015;Tang et al., 2012).

Supply and demand
As the fundamental factor, supply and demand have been the main factors affecting oil prices. Supply and demand changes have always been the fundamental factors affecting the long-term trend of oil prices. (Hamilton, 2009b) analyzed the drivers of oil prices and argued that the main reason for the rise in oil prices in 2007-2008 was the global demand for production. (Kilian, 2009) developed a structural VAR model to explain the global crude oil price fluctuation and understand the reaction of the US economy associated with oil price volatility. The crude oil price was decomposed into three components: crude oil supply shock, the shocks to the global demand for all industrial commodities, and the demand shock to the global crude oil market. However, in recent years, with the development of alternative energy sources, the worldwide supply and demand structure of crude oil has changed. (Kilian, 2017) reported the increased U.S. tight oil production not only reduced demand for oil in the rest of the world and lowering the Brent oil price but also caused other countries to cut back on their oil imports, lowering global oil demand.

Global economic development
Global economic development is a manifestation of supply and demand (Doroodian & Boyd, 2003;Sadorsky, 1999;Barsky & Kilian, 2001). (Kilian & Hicks, 2013) measured the global demand shock directly by correcting the real gross domestic product (GDP) growth forecast. The results showed that the forecast was associated with unexpected growth in emerging economies during the 2003 to 2008 period. These surprises were associated with a hump-shaped response of the real price of oil that reaches its peak after 12-16 months. The global real economic activity has always been considered to impact the changes in oil price significantly. (Özbek & Özlale, 2010) researched the relationship between global economic and oil prices with trend and cycle decomposition. They found that economic shock has a lasting effect on oil prices, which were considered mainly to be supply-side driven.

Financial factor
In addition to commodity attributes, crude oil also has financial attributes. The longterm trend of crude oil price is determined by the commodity attributes, which are affected by the supply and demand factors generated by the real economy; the shortterm fluctuations of crude oil price are determined by the financial attributes, which are influenced by market expectations and speculative transactions. The financial factor mainly includes speculation factor, exchange rate and some other financial index, which connect the stock market and monetary market with the crude oil price (Narayan et al., 2010;Zhang, 2013;Reboredo, 2012;Cifarelli & Paladino, 2010). (Kilian & Murphy, 2014) developed a structural model to estimate the speculative component of oil price through the inventory data and found it played an important role during earlier oil price shock periods, including 1979including , 1986including and 1990including . (Sari et al., 2010 estimated the comovement and information transmission among oil price, exchange rate and the spot prices of four precious metals (gold, silver, platinum, and palladium). Investors could diversify their investment risk by investing in precious metals, oil, and euros.

Technology factor
The Crack spread is defined as the price difference between crude oil and its refined oil, reflecting the supply and demand relationship between the crude oil market and its refined product market . (Murat & Tokat, 2009) used the random walk model (RWM) as a benchmark to compare the crack spread futures and crude oil futures and found the crack future could forecast the movements of oil spot price as reasonable as the crude oil futures. (Baumeister et al., 2013) selected crack spread as one of the variables to forecast crude oil prices, and the studies suggested it was an influential prediction factor.

Forecast method
Except for the influence factors, researchers are also very concerned about the forecast methods for improving forecast accuracy. The four main forecast method categories: time series models, econometric models, qualitative methods and artificial intelligence techniques are used in oil price modeling and forecasting (Wang et al., 2016;Charles & Darné, 2017;Yu et al., 2015;Sun et al., 2019;Suganthi & Samuel, 2012;Zhang et al., 2008;Valgaev et al., 2020). The autoregressive integrated moving average (ARIMA) and exponential smoothing (ETS) are the most widely used time series forecasting model, and they are usually used as the benchmark models (Wang et al., 2018;Chai et al., 2018;Zhu et al., 2017). In addition, the econometric models and qualitative methods like the generalized autoregressive conditional heteroskedastic model (GARCH), the vector autoregression model (VAR), the state-space models and the threshold models are also widely used (Kilian, 2010;Wang & Sun, 2017;Zhang & Wei, 2011;Ji & Fan, 2016;Drachal, 2016).
However, with the increasing of the data volume and influence factors complex, traditional models failed in predicting accurately. The machine learning forecasting method presents its superiority and mostly outperform traditional approaches when tested with empirical results, especially in dealing with the nonlinear problem and short-term prediction. Such as support vector machines (SVM), artificial neural networks (ANNs), genetic algorithms (GA) and wavelet analysis are introduced into oil price forecasting in recent years. For example, (Zhao et al., 2017) proposed the stacked denoising autoencoders model (SDAE) for oil price forecasting. Empirical results of the proposed method proved the effectiveness and robustness statistically. (Xiong et al., 2013) developed an integrated model EMD-FNN-SBN, which is the empirical mode decomposition (EMD) based on the feed-forward neural network (FNN) modeling framework incorporating the slope-based method (SBM). The results indicate this model using the (multiple-input multiple-output) MIMO strategy is the best in prediction accuracy.
During the last decades, more and more factors and models have been introduced, estimated and validated. Several different factors can address the oil price forecasting problem from the empirical and theoretical vision. Many researchers always select general factors and models directly, regardless of which indicators are the actual core variables. Especially with the expansion of data and quantitative indicators, variable selection becomes more and more critical. In recent years, there are some papers begin to extract core factors before forecasting. Even though there are some variable selection processes in some machine learning methods, they are all nested in the forecasting and just for the robustness of the model (Drezga & Rahman, 1998;May et al., 2008;Korobilis, 2013;Huang et al., 2014). There are fewer papers devoted to variable selection before predicting. For example, (Chai et al., 2018) used the Bayesian model average method for influence variable selection before establishing the oil price forecasting model. (Zhang et al., 2019) accurately screen out the main factors affecting oil price by using an elastic network model and LASSO shrinkage in the case of many predictive variables but relatively few data. The main factors influencing oil price forecast are studied from the Angle of variable selection. Secondly, the accuracy and robustness of the elastic network model and the LASSO contraction method in predicting oil prices are comprehensively verified using a variety of robustness tests. The results show that the LASSO contraction and elastic network model outperforms other standard oil price forecasting models. An investor who allocates assets based on the predictions of these two methods can achieve a more substantial return than other oil price forecasting models.
In this paper, we develop an integrated model with a new machine learning method for crude oil price forecasting based on core factor selection. This paper contributes to the variable selection and machine learning method in oil price forecasting. In the process of variable selection, we introduce three approaches with different advantages for comparison analysis and forecasting, elastic-net regularized generalized linear model, Bayesian model average and spike-slab lasso method. In addition, we combine them with a new machine learning method long short-term memory (LSTM) model for oil price forecasting. Finally, random walk (RW), autoregressive integrated moving average models (ARMA), elman neural Networks (ENN), ELM Neural Networks (EL), Walvet Neural Networks (WNN) and generalized regression neural network (GRNN) Models were used to forecast the price. Finally, we compare and analyze the different results with root mean squared error (RMSE), mean absolute percentage error (MAPE), directional symmetry (DS). The research framework is shown in Fig.1.
The structure of the paper is as follows: Section 1 reviews the related literature including influencing factors and forecast methods. Section 2 introduces the data. Section 3 develops the technique. Section 4 presents the empirical analysis. Finally, Section 5 concludes the paper and outlines future work.

Dataset
According to the above literature, we selected 30 variables from supply and demand, inventory, financial market, technical aspects as the initially chosen variables (Hamilton, 2009a;Kilian & Murphy, 2014;Zhang et al., 2017;Wang et al., 2015;Tang et al., 2012), and we choose the real monthly West Texas Intermediate (WTI) crude oil price as the dependent variable. The interval of the data is from January 2000 to December 2017. For data stability, we used the return rate of data. The crude oil production, consumption structure and replacement cost as the related variables measure the supply index. The demand index includes crude oil consumption and global economic development, a measure of real global economic activity.
Meanwhile, we select the total OECD petroleum inventory, U.S. crude Oil inventory (total, SPR, and Non-SPR) as the inventory index. In addition, the related variables are selected from speculation, monetary, stock, commodity market as the financial factor index. Finally, we calculated the WTI-Brent spot price spread, actual value of the WTI crack spread and Brent crack spread: actual value as the technical indicators. In Table 1, we describe each variable and its corresponding data sources.

Theoretical background
As the crude oil market is very complex and has various uncertain determinants, we must select the core influence factors first before establishing forecasting models. The main four variable selection methods are significance test (forward and backward stepwise regression), information criteria (AIC BIC), principal component factor analysis model, lasso regression, ridge regression, and other punitive models (Castle et al., 2009). It is hard to tell which the best is because each has its own strong and weak points. Thus, we introduce three different methods to select core influence factors of crude oil price, which are elastic-net regularized generalized linear Models (GlMNET), spike-slab lasso method (SSL) and Bayesian model averaging (BMA). These three methods are effective variable selection methods and they are all improvements on the existing mature models (LASSO, Ridge regression). Thus, we use these new methods for variable selection (Zou & Hastie, 2005;Friedman et al., 2010).

Variable selection
The elastic-net regularized generalized linear Models (GLMNET) Zou and Hastie (2005) (Zou & Hastie, 2005) proposed the elastic-net method for variable selection, which is considered to be the best contraction method for handling multicollinearity and variable screening, and its loss precision is not too great. Their simulation results showed that the elastic net outperformed the Lasso in terms of prediction accuracy. Like the Lasso, the elastic net simultaneously does automatic variable selection and continuous shrinkage, and it can select groups of correlated variables. On the one hand, it achieves the purpose of ridge regression to select essential features; on the other hand, it removes features that have little influence on dependent variables, like Lasso regression, and achieves good results, especially when the sample size n is smaller than the number of predictors. (The specific formula refers to (Zou & Hastie, 2005)) In this paper, we choose the Elastic-Net Regularization Linear Model (GlMNET), which is a package that fits the Generalized Linear model by punishing maximum likelihood (Friedman et al., 2010). The regularization path of the Lasso or elastic net penalty is calculated on the value grid of the regularization parameter. The algorithm is high-speed, can make full use of the sparsity of input matrix X, and is suitable for linear, logic, polynomial, poisson and Cox regression models. It can also be applied to multi-response linear regression. These algorithms can process vast datasets and can make use of sparsity in the feature set.
Wherein the values of the grid λ cover the entire range, l(y i , η i ) is the negative logarithmic likelihood distribution of the contribution to the observed value i. For example, it is 1 2 ðy−ηÞ 2 for Gaussian. The elastic mesh penalty is controlled by α. (The specific formula refer to Friedman et al. (2010)).

Bayesian model averaging
The basic idea of the Bayesian model averaging approach (BMA) can comprehend. Each model is not fully accepted or not entirely negative (Leamer, 1978). The prior probability of each model should be assumed firstly. The posterior probability can be gain by extracting the dataset contains information as well as the reception of models to the dependent variables. The excellence of the BMA approach is not only that it can sort the influence factors according to their importance but also can calculate the posterior mean, standard deviation and other indicators of the corresponding coefficients. With the help of the Markov chain Monte Carlo method (MCMC), the weight distribution of the model according to the prior information could be estimated (Godsill et al., 2001;Green, 1995). The MCMC method can overcome the shortcoming of BIC, AIC and EM methods. (The specific formula refer to (Leamer, 1978;Merlise, 1999;Raftery et al., 1997)). It has the following three advantages: First, under different conditional probability distributions, there is no need to change the algorithm. Second, the posterior distribution of the weight and variance of BMA is considered comprehensively. Third, it can handle parameters with high BMA correlation.

Spike-slab lasso
Although model averaging can be considered a method of handling the variable selection and hypothesis testing task, only in a Bayesian context since model probabilities are required. Recently, Ročková, V. and George, E. I. introduce a new class of selfadaptive penalty functions that moving beyond the separable penalty framework based on a fully Bayes spike-and-slab formulation (Ročková & George, 2018). Spike-slab Lasso (SSL) can borrow strength across coordinates, adapt to ensemble sparsity information and exert multiplicity adjustment by non-separable Bayes penalties. Meanwhile, it is using a sequence of Laplace mixtures with an increasing spike penalty λ 0 , and keeping λ 1 fixed to a small constant. It is different from Lasso, which a sequence of single Laplace priors with an increasing penalty λ. Furthermore, it revisits deployed the EMVS procedure (an efficient EM algorithm for Bayesian model exploration with a Gaussian spike-and-slab mixture (Ročková & George, 2018)) for SSL priors, automatic variable selection through thresholding, diminished bias in the estimation, and provably faster convergence. (The specific formula refer to (Ročková & George, 2018)).

Long short-term memory network
Long short-term memory (LSTM) neural networks are a special kind of recurrent neural network (RNN). LSTM was initially introduced by Hochreiter and Schmidhuber (1997) (Hochreiter & Schmidhuber, 1997), and the primary objectives of LSTM were to model long-term dependencies and determine the optimal time lag for time series issues. In this subsection, the architecture of RNN and its LSTM for forecasting crude oil prices are introduced. We start with the primary recurrent neural network and then proceed to the LSTM neural network.
The RNN is a type of deep neural network architecture with a deep structure in the temporal dimension. It has been widely used in time series modeling. A traditional neural network assumes that all units of the input vectors are independent of each other. As a result, the conventional neural network cannot make use of sequential information. In contrast, the RNN model adds a hidden state generated by the sequential information of a time series, with the output dependent on the hidden state. Figure 2 shows an RNN model being unfolded into a full network. The mathematical symbols in Fig. 2 are as follows: 1. x t denotes the input vector at time t.
2. s t denotes the hidden state at time t, which is determined relayed on the input vector x t and the previous hidden state. Then the hidden state s t is determined as follows: is the activation function, it has many alternatives such as sigmoid function and ReLU. The initial hidden state s 0 is usually initialized to zero.
3. o t denotes the output vector at time t. It can be calculated by: 4. U and V denote the weights of the hidden layer and the output layer respectively. W denotes transition weights of the hidden state. Although RNNs simulate time series data well, these are difficult to learn long-term dependence because of the vanishing gradient issue. LSTM is an effective way to solve the vanishing gradient by using memory cells. A memory cell is consists of four units: input gate, forget gate, output gate and a self-recurrent neuron, it is shown in Fig. 3. The gate controls the interactions between the adjacent memory cells and the memory cell itself. Whether the input signal can change the state of the memory cell is controlled by the input gate. On the other hand, the output gate can control the state of the memory cell to decide whether it can change the state of other memory cells. Additionally, the forget gate can choose to remember or forget its previous state. Figure 4 illustrates the unrolled module in an LSTM network, which describes how the values of each gate are updated. The mathematical symbols in Fig. 4 are as follows: 1. x t is the input vector of the memory cell at time t.
6. f t and C t are values of the forget gate and the state of the memory cell at time t, respectively. f t and C t can be formulated as follows: . o t and h t are values of the output gate and the value of the memory cell at time t, respectively. o t and h t can be calculated as follows: The architecture of the LSTM neural network includes the number of hidden layers and the number of delays, which is the number of past data that account for training and testing. Currently, there is no rule of thumb to select the number of delays and hidden layers. In this work, the number of hidden layers and delays are set to 5 and 4 by trial and error. The back-propagation algorithm is employed to train the LSTM network and MLP neural network. The learning rate, batch size and number of epochs are 0.05, 60 and 1000, respectively. The speed of convergence is controlled by the learning rate, which is a decreasing function of time. Setting the number of epochs and the learning rate to 1000 and 0.05 can achieve the convergence of the training. The empirical result will become stable once convergence is achieved through the combinations of parameters are varied. Interesting readers may refer to (Hochreiter & Schmidhuber, 1997) for more information.

Empirical study
In this section, we compare the variable selection-LSTM integrated learning approach to the predictive performance of some benchmark models. First, section 4.1 analyzes the core influencing factors for the screening of the three feature extraction methods, and 4.2 describes the evaluation criteria and statistical tests to compare prediction accuracy. Second, section 4.3 provides an input selection and reference model parameter settings. Finally, Section 4.4 for the discussion.

Variable selection
We can see from Table 2, the elastic-net selects the most number (18 variables), followed by the SSL method (11 variables), and the BMA method includes the least number (8 variables). Meanwhile, the variable of the SSL method is a subset of the BMA method, and the BMA method is a subset of the elastic-net. Non-OPEC production, shale oil (tight oil) production, Fed Fund effective, Kilian Index, USA CPI, USA PPI: Energy, Euro PPI, USA: PMI, OECD inventory, USA SPR inventory, USA Non-SPR inventory, Crude oil non-commercial net long ratio, Real dollar index: generalized, COMEX: Gold: Future closing price, LME: Copper: Future closing price, WTI-Brent spot price spread, WTI crack spread: actual value and Brent crack spread: real value. Firstly, we can see Non-OPEC production and tight oil production were selected from the supply aspect. It suggested that with the reduction of OPEC production, the production capacity of Non-OPEC countries is increasing gradually. In particular, the US tight oil production has become an essential factor affecting the trend of oil prices in recent years. According to the IEA report, the global oil production is expected to increase by 6.4 million barrels to 107 million barrels by 2023. The US tight oil production growth will account for 60 of the global growth. Meanwhile, in 2017, the US was the world's largest producer.
From the demand aspect, global economic development is still the main driver of crude oil price. The USA PPI: energy, Euro PPI and USA: PMI factors are selected by all three methods. Crude oil is an important raw material for industry, agriculture, and transportation. It is also the main parent product of energy and chemical products in the middle and lower reaches. Therefore, crude oil plays a critical role in the price of domestic production and living materials. For example, when the oil price continued to fall sharply, which is bound to lower the overall price level of the country. Moreover, it will have a heavier negative effect on domestic currency fluctuations, which will shift the currency situation from expansion to contraction.
From the inventory aspect, the OECD inventory, the USA SPR inventory and the USA Non-SPR inventory are chosen into the elastic-net model. The inventor is an indicator of the balance of the supply and demand for crude oil. Furthermore, the impact of commercial inventory on oil prices is much more substantial. When the future price Table 2 Selected key features by GLMNET, SSL and BMA method

Methods
Selected feature ID GLMNET X 2 , X 5 , X 8 , X 9 , X 10 , X 13 , X 14 , X 15 , X 16 , X 18 , X 19 , X 20 , X 21 , X 26 , X 27 , X 28 , X 29 , X 30 Spike-slab Lasso X 13 , X 14 , X 15 , X 20 , X 21 , X 27 , X 29 , X 30 Bayesian model averaging X 13 , X 14 , X 15 , X 16 , X 18 , X 19 , X 20 , X 21 , X 27 , X 29 , X 30 is much higher than the spot price, the oil companies tend to increase the commercial inventory, which stimulates the spot price to rise and reduces the price difference. When the future price is lower than spot prices, oil companies tend to reduce their commercial inventories, and spot prices fall, which will form a reasonable spread with futures prices. From the speculate aspect, there is a positive correlation between the noncommercial net long ratio and oil price (Sanders et al., 2004). With the crude oil market and the stock market relation gradually strengthened, hedging plays a more important role in driving market trading (Coleman, 2012).
From the exchange rate market, the real dollar index: generalized was selected. The dollar index is used to measure the degree of change in the dollar exchange rate against a basket of currencies. If the dollar keeps falling, real revenues from oil products priced in dollars will fall, which will lead to the crude price high.
Form the commodity market, the LME: Copper: Future closing price factor is picked up. Copper has the function of resisting inflation, while the international crude oil price is closely related to the inflation level. There is an interaction between them from the long-term trend.
From the technical aspect, the WTI-Brent spread serves as an indicator of the tightening in the crude oil market. As the spread widens, which suggests the global supply and demand may be reached a tight equilibrium. The trend of price spread showed a significant difference around 2015 because of the US shale oil revolution. US shale production has surged substantially since 2014. However, due to the oil embargo, the excess US crude oil could not be exported, resulting in a significant increase in US Cushing crude oil inventories. The spread at this stage was mainly affected by the WTI price, and the correlation between the price difference and the oil price trend was not strong. After the United States lifted the ban on oil exports at the end of 2015, the correlation between the price spread and the oil price trend increased significantly. After the lifting of the oil export ban, the WTI oil price was no longer solely affected by the internal supply and demand of the United States. During this period, Brent and WTI oil prices were more consistent, resulting in a narrower spread. In addition, after 2015, the consistency of them increased significantly.
In summary, the factors selected by all three models are USA PPI: Energy, Euro PPI, USA: PMI (global economic development), Crude oil non-commercial net long ratio (speculate factor), Real dollar index: generalized (exchange rate market), LME: Copper: Future closing price (commodity market), WTI crack spread: actual value and Brent crack spread: actual value (technology factor).

Evaluation criteria and statistic test
To compare the forecasting performance of our proposed approach with some other benchmark models from level forecasting and directional forecasting, three main evaluation criteria, i.e., root mean squared error (RMSE), mean absolute percentage error (MAPE), directional symmetry (DS), which have been frequently used in recent years (Chiroma et al., 2015;Mostafa & El-Masry, 2016;Yu et al., 2008b;, are selected to evaluate the in-sample and out-of-sample forecasting performance. Three indicators are defined as follows: where x t andx t denote the actual value and forecasted value at time t, respectively, To provide statistical evidence of the forecasting performance of our proposed ensemble learning approach, three tests, i.e., the Diebold-Mariano (DM) (Diebold & Mariano, 1995) test, the Superior Predictive Ability (Hansen, 2005) test and Pesaran-Timmermann (PT) (Pesaran & Timmermann, 1992) test, are performed. The DM test checks the null hypothesis of equal predictive accuracy. In this study, the Mean Squared Error is applied as DM loss function and each model is compared against a random walk. As for performance measurement, we use the RMSE and MAPE. The PT test examines whether the directional movements of the real and forecast values are the same. In other words, it checks how well rises and falls in the forecasted value follow the actual rises and falls of the time series. The null hypothesis is that the model under study has no power in forecasting the oil prices.

Forecasting performance evaluation
To verify the predictive power of the variable Select-machine learning integrated approach, we selected 8 benchmark models, including 6 multivariate models (MLP, RBFNN, GRNN, ELMAN, WNN, ELM) and 2 univariate models (RW and ARMA). In this paper, the number of MLP output layer neurons is 1, the number of iterations in the training stage is 10,000, and the number of hidden layer neurons is determined by the trial and error method to be 11. Similarly, the number of LSTM hidden layers and the number of delays were set as 5 and 4, respectively, and the number of output layer neurons was set as 1. The structure of the LSTM neural network was trained by backpropagation algorithm (BP), and the learning rate, batch size and the number of the epoch were set as 0.05, 60 and 1000 respectively. The convergence rate is controlled by the learning rate, which is a function of decreasing time. When the number and learning rate of epochs is set at 1000 and 0.05, the convergence of the training set is realized, and the empirical results tend to be stable, which can recognize the convergence of the training set data. When the parameter combination changes, once it converges, the experimental results tend to be stable. All models are implemented using Matlab 2017B software.
According to the forecast results of Table 3, we can find some interesting conclusions: (1) no matter in the sample inside or outside the sample prediction, this chapter proposed variable selection -machine learning approach to integration in the training set and test set level precision (RMSE and MAPE) and direction (DS) were better than that of the single variable precision model and the core factors extracted model. (2) Among the variable selection-machine learning integration models, the BMA-LSTM integration model performs best, followed by Spike and Slab LASSO-LSTM and GLMN ET-LSTM. For example, the RMSE, MAPE and DS predicted in the SAMPLE of rhe BMA-LSTM integrated module were 1.12%, 0.74% and 84.88%, respectively, which were 3.77%, 3.28% and 35.46% higher than RW, and 1.09%, 0.61% and 12.21% higher than the LSTM model without variable selection. (3) When predicted one step in advance, RMSE, MAPE and DS of the BMA-LSTM integrated model were 2.04%, 0.83% and 81.40%, respectively, 3.7%, 4.36% and 32.56% higher than that of the RW model, and 1.13%, 0.73% and 11.63% higher than that of the variable free LSTM model. It shows that the prediction accuracy of the variable selection-machine learning integrated model is significantly improved compared with that of the univariate model and the univariate model. Secondly, the number of core variables selected by BMA is neither the most nor the least among the three variable selection models, indicating that the number of core variables will also affect the prediction results.

Statistic tests
According to Table 4 statistical test results can be seen that (1) the step ahead prediction samples, variable selection -machine learning integration model of DM test results are less than 7.195, this means that the performance of the proposed method under the confidence level of 99% is better than all the other benchmark model, the possible reason is the variable selection -machine learning integration model significantly improves the prediction performance of the model. (2) In the out-of-sample prediction 1 step in advance, when the LR model is used as the test model, the DM test results of other test models are far less than − 7.015, indicating that the predictive performance of the three integrated methods and the other three single models is better than that of the machine learning model without variable selection at the 99% confidence level.
(3) samples within 1 step ahead prediction and sample 1 step ahead prediction of the performance of the three variables extraction method was compared, the BMA-LSTM integration model to predict performance is the best, the next step is the Spike and slab LASSO-LSTM and GLMNET-LSTM, suggesting that this chapter puts forward the integrated research framework based on the variable selection-machine learning significantly improves the performance of the integrated machine learning approach. PT test results also give three interesting points: (1) In the one step forward insample forecasting, the PT test results of the proposed variable selection-machine learning integration approach are all rejected the movement direction of the actual independence assumption under 99% confidence level. This also means that the variable selection-machine learning method is the best direction prediction performance and also can be seen that the direction of the ARIMA predicts performance is the worst. (2) In the out-of-sample one-step forecasting, the PT test value of the predicted results of the integrated method is significantly greater than that of the single model, which means that the direction prediction ability of the integrated method is better than that of the single model, mainly because the variable selection-machine learning integration idea significantly improves the direction prediction performance of the single model.
(3) In the prediction in and out of sample 1 step in advance, it can be seen from the PT test results of variable Selection-machine learning integration method that the direction prediction accuracy of BMA-LSTM is the highest, followed by Spike-Slab Lasso-LSTM, which is mainly attributed to the direction prediction ability of variable Selection-machine learning method.

Conclusions and future work
In this paper, we proposed a variable selection and machine learning framework that combines the variable selection (BMA) and forecasting method (LSTM) to forecast the oil price and compared its forecasting performance with other primary and new variable selection methods (elastic-net and spike and slab Lasso). Moreover, compared to other popular benchmark forecast methods (RW, ARMA, MLP, RBFNN, GRNN, ElMAN, WNN, ELM). Specifically, our contributions are as follow: Introduce the variable selection before forecasting. In this process, we compare three different methods and analyze core influencing factors based on the literature review from supply and demand, global economic development, financial market, and technology aspects. The results showed that the variable of the SSL method is a subset of the BMA method, and the BMA method is a subset of the elastic-net.
Testing the performance of the proposed variable selection and machine learning framework based on 3 variable selections and 8 individual forecasts. Comparing with the 8 individual forecasts without variable selection, the combinations forecasting reduces the errors. The results showed that (1) the variable choice-machine learning integration method proposed in this chapter is superior to the univariate model and the model without core factor extraction in both training set and test set level accuracy (RMSE, MAPE) and direction symmetric (DS). (2) Among the variable selectionmachine learning integration models, The BMA-LSTM integration model performs best, followed by Spike and Slab LASSO-LSTM and GLMNET-LSTM. It shows that the prediction accuracy of the variable selection-machine learning integrated model is significantly improved compared with that of the univariate model and the univariate model. Secondly, the number of core variables selected by BMA is neither the most nor the least among the three variable selection models, indicating that the number of core variables will also affect the prediction results. (3) The statistical test results show that the prediction of 1 step in advance in-sample and 1 step in advance in out of sample. Compared with the prediction performance of the three variable extraction methods, the directional prediction accuracy and horizontal prediction accuracy of the BMA-LSTM integrated model are the best, followed by Spike and Slab-LASSO-LSTM and GLMNET-LSTM. This indicates that the variable selection-based machine learning integrated research framework proposed in this chapter significantly improves the forecasting performance of oil prices. In future research, we may introduce more independent variables with the help of internet search data, test our framework performance. Moreover, investor sentiment can be quantified in this process. In addition, different variable selection methods can be introduced more.