Computers, Materials & Continua
DOI:10.32604/cmc.2021.012302
images

Article

Improving Stock Price Forecasting Using a Large Volume of News Headline Text

Daxing Zhang1,* and Erguan Cai2

1Department of Mathematics, Clemson University, Hangzhou, 310018, China
2Institute of Graphics and Image, Hangzhou Dianzi University, Hangzhou, 310018, China
*Corresponding Author: Daxing Zhang. Email: dxzhang@hdu.edu.cn
Received: 30 August 2020; Accepted: 20 May 2021

Abstract: Previous research in the area of using deep learning algorithms to forecast stock prices was focused on news headlines, company reports, and a mix of daily stock fundamentals, but few studies achieved excellent results. This study uses a convolutional neural network (CNN) to predict stock prices by considering a great amount of data, consisting of financial news headlines. We call our model N-CNN to distinguish it from a CNN. The main concept is to narrow the diversity of specific stock prices as they are impacted by news headlines, then horizontally expand the news headline data to a higher level for increased reliability. This model solves the problem that the number of news stories produced by a single stock does not meet the standard of previous research. In addition, we then use the number of news headlines for every stock on the China stock exchange as input to predict the probability of the highest next day stock price fluctuations. In the second half of this paper, we compare a traditional Long Short-Term Memory (LSTM) model for daily technical indicators with an LSTM model compensated by the N-CNN model. Experiments show that the final result obtained by the compensation formula can further reduce the root-mean-square error of LSTM.

Keywords: Deep learning; recurrent neural network; convolutional neural network; long short-term memory; stocks forecasting

1  Introduction

The price changes in financial markets are nonlinear and the influencing factors are variable. Stock markets attract many wealthy shareholders and researchers. Henrique et al. [1] study mentions a theory called efficient-market hypothesis (EMH), arguing that because financial markets will immediately adapt to any information available, people cannot predict the market’s movements. In many studies, securities analysts research the price changes in the stock market, then analyze this vast amount of data and come to useful conclusions. Professionals in this field invest a good deal of time in learning how to analyze data. Artificial intelligence technology is increasingly being incorporated in data analysis, particularly in the area of predicting stock movements.

Stock price forecasting methods can be divided into two categories: a time series method that was modeled by Aue et al. [2] using a process called autoregressive conditional heteroscedasticity (ARCH); and machine learning algorithms. As of 2018, models based on artificial neural networks (ANNs) and support vector machines (SVMs) were still at the core of a stock market’s analysis. ANNs contain many neurons, can handle the relationship between large amounts of data, and have excellent performance with financial time series data [3,4]. Multilayer feedforward networks (also called the BP network model) were used for time series data prediction [5]. Neural networks continue to improve. Convolutional neural networks (CNNs) and recurrent neural networks (RNN) offer good performance in a specific mode. Long short-term memory (LSTM) represents an improvement over RNNs and is considered to be the most suitable network for predicting stock data [6]. However, Kim et al. [7–9] expressed data and indicators of stock as a chart, leveraged CNN, and proposed a CNN-LSTM model, proving that CNN still plays an important role in the research of predicting stock prices.

LSTM inputs time series data for the purpose of prediction. Most stocks have more than 6,000 fundamentals, which means that technical analysis (TA) indicators have the same data sample. On the basis of a time series, news related to daily fluctuations in stock prices can also be part of another CNN algorithm input. Ding et al. [10] shows that shareholders can view stock-related news positively or negatively, which will determine whether they buy or sell stocks, thereby affecting the fluctuation of stock prices.

Improved algorithms related to LSTM all use fundamental indicators and TA indicators as input, but differ in the length of the predicted period. Rather et al. [11] concentrated on every five minutes when a stock price changes, Chung et al. [12] predicted daily stock price, and Weng et al. [13,14] predicted the price of stocks one month out for major U.S. stock and sector indices. However, daily price research is the most common time period. For the input dimension of daily price data, more data represents more information, which can reduce the likelihood of errors in a prediction.

Figure 1: Increasing dataset size over time

Ding et al. [15] showed that no matter whether the news is weekly or monthly, it affects the stock market trend of the next day. Therefore, this study uses two weeks’ worth of news headlines as data. However, when they did related research, they only tracked the news data of current stocks, which resulted in an insufficient sample size for deep learning algorithms, and the final experimental results were not very promising. When using a supervised deep learning algorithm, at least 5000 of each type of labeled sample is required to reach an acceptable range. When the number of samples is 10 million, it is generally sufficient to surpass human performance. Goodfellow et al. [16], mentioned in Fig. 1 below, shows the amount of sample data that emerged from deep learning in 2015. The results of deep learning algorithms are positively correlated with the amount of data. While tracking a single stock, even when it starts to record news from the time a company goes public, most companies in China can only possess a sample of around 5,000. Emerging companies do not generate enough news samples, and so an emerging company cannot guarantee enough weight for an algorithm to predict the future price of stock. To expand the amount of stock news sample data, this paper unifies all stock news that can be collected, removes redundant information, improves the accuracy of experimental results, and expands the use of the algorithm to stock prediction.

2  Materials and Methods

This section describes the sources of the datasets used in this paper and distributions to each network model. There were two input types: TA indicators, which display daily financial technical directional-movements series called Stock datasets, and half-month financial news titles of each stock, called News datasets.

2.1 Stock Datasets

The basic stock data were obtained from Tonghuashun (China’s professional economic website), marked as xS.

Web crawler technology was used to crawl daily data and TA indicators from the website. Data on 1014 individual stocks were obtained from listing up to 3/8/2019, including Open, Close, High, Low, Volume, Vibration, as LSTM first layer input, with 80% of stocks representing the training set, and 20% the test set. The calculation of TA indicators is similar to that in Guresen’s [4].

The majority of papers predict or calculate the composite index as their main research target, such as the Shanghai Stock Exchange (SSE) Composite Index, which makes result more stable and more representative. The LSTM input indicators dataset is the largest one. The SSE Composite Index contains all the stocks listed on the SSE. In terms of time, the prices of stocks on the SSE tend to be more affected by political factors than ordinary events. The bigger the news, the greater the impact on the index [10]. However, the prices of individual stocks are easily influenced by significant company events. Therefore, the daily basic data of the SSE Composite Index Xs′ experimental results are only used as a sample comparison with individual stocks on LSTM. This is why this paper also focuses on the effect of deep learning on individual stocks.

Data xS was used to prove the link between the composite index and individual stock data in LSTM. The training model of all 1014 stocks can be used to predict the final stock price fixed by news trained model in last formula. The results can further reduce the number of errors of LSTM.

2.2 News Datasets

News headlines text were also obtained from Tonghuashun. We call this data xN.

Ding’s algorithm uses about 100 news headlines and content in order to predict each single stock price. Deep learning algorithms are often dependent on big data. If the data volume is low, the final result will be unsatisfactory for the LSTM algorithm. In Ding’s paper, the highest accuracy achieved by the algorithm is only 56%. The easiest way to increase accuracy of stock news on stock price forecasting algorithms is to increase the amount of text data. This paper proposes ideas for achieving this target volume.

Existing research tracked only single stock news series as input for calculating a specific stock price. That’s why the CNN net input dataset number is small. We think that, under normal circumstances, different stocks have different sensitivities to similar news, especially financial news. The price trends of A-shares will be stable even if the news said “Huge Funds Coming Soon,” but B-shares are more likely to be affected. A single stock may only produce 200 news stories in an entire year. To improve the final accuracy of using stock headlines for stock forecasts, this paper makes the assumption that each stock has exactly the same impact on similar news, and gathers all stocks’ news headlines within 15 days to calculate the next day’s price fluctuations. This method can improve the accuracy of the algorithm because of the large amount of data, while ignoring the negative effects caused by different stock sensitivity. In the Chinese stock market, there are now 3000 stocks. This means the number of news headlines for a single stock can be expanded by 3000 times. If a stock’s news is tracked within half month, the amount of sample data that can theoretically be news headlines could reach 1 million. The experiment described in Section 3 of this paper also studies the impact of news headline text data on the accuracy of the CNN.

In summary, this paper shifts from research on vertical data to research on horizontal data. All stocks’ price impact factors are considered the same, and stock headline data is horizontally enlarged at the same time. Unfortunately, this paper only uses the news headlines of 1014 stocks vs. all 3000 stocks in the Chinese stock market, and the amount of data is therefore expanded by 1014 times. Detailed data when tracking each stock’s news spans within 15 days, until 5/23/2019, and the news’s time limit on May 23rd, for example, is for the closing time of 3 pm, which is used to predict the highest price of May 24. This assumption is feasible if it can increase the amount of data expansion in the CNN in order to improve final accuracy. To further lessen the impact of different companies on the news sensitivity, after the text data is unified, the redundant text should be reduced. It has been observed that many news headlines contain names of companies and/or stocks. These names are not valid information for the actual algorithm, because this type of information is deleted in text preprocessing. For the 1014 stocks, there were 29,700 news headlines. Therefore, every stock had 29.3 news headlines on average. The training and test set settings are shown in Tab. 1. The training forecast set allocation ratio is the same as the stock's fundamental indicator dataset, and the news headlines are obtained. Of the text in the dataset, 80% is used for training, and the remaining 20% is used as a test set.

To verify the final network effect, we should re-establish verification set outside the time points of data xS and xN, so we also get stock base data from the 23rd (Thursday) and the following day. The highest price is calculated as the accuracy of the label layer compared to previous day’s highest price. The higher price on the second day is set to 1 in the label layer, and vice versa, the lower price is set to 0, which predicts the stock price of the following day, all the way through 15 days of news headlines. We call this data news of xF, which contains the TA indicators of Stock Data LSTM (D-LSTM), and also contains News Data CNN (N-CNN)'s news headlines text. The final experiment will take the next day's highest price H from D-LSTM and H′ contrast corrected by a compensation formula in which parameters are obtained from N-CNN.

2.3 Nets Design

2.3.1 N-CNN Design

Fig. 2 shows the design of the CNN for news. First, data language type obtained from news headlines is Chinese. An embedding layer is needed to convert text into a word vector. In this step, we use the Word2vec tool and the continuous Bag of Words (CBOW) model for the conversion of Chinese word embedding.

Figure 2: News CNN model architecture

The output of a word vector is a two-dimensional matrix, which is used as input of the convolutional layer. The convolutional layer is often followed by the pooling layer to reduce dimension, the number of parameters, and training time. Immediately after the pooling layer is a three fully connected (FC) layers. The third FC layer activation function is softmax, whose formula is shown as Eq. (1). Since we only need to convert stock-derived information into a rising or falling output, Softmax outputs only two types, a label that is 0 or 1. Score set S calculated by the pre-neuron is mapped by formula to interval (0, 1). The result is [O1, O2], where O1 represents the probability of falling, and O2 represents the probability of rising.

Oi=eSi∑jeSj (1)

2.3.2 LSTM Introduction

A recurrent neural network (RNN) differs from a traditional neural network in that it can consider the influence of information that has already appeared on the next prediction. LSTM performs better than the other RNNs [17] in long-term sequence data. Unlike an RNN that simply uses the hyperbolic tangent function (tanh) layer as the structure of repeating module, LSTM has more interactive layers, as shown in Fig. 3.

Eqs. (2) to (5) show calculations for each gate and cell state, where it, ft,c~t, ot are all sigmoid layers expressed by σ(⋅),

output=σ(W⋅[x+h]+b) (2)

c~t=tanh⁡(Wcxt+Whht−1+bc) (3)

ct=ft∗ct−1+it∗c~t (4)

ht=ot∗tanh⁡(ct) (5)

W represents weight matrices and b is a bias term. The cell’s workflow is as follows: xt is current input and ht−1 is previous input. The first step is the forget gate deciding to discard the information, the σ(⋅) function only outputs value between 0 and 1, with 1 completely reserved and 0 completely discarded. Through this door, we have deleted information, and then the input gate has to decide to add some information. In addition to the same σ layer, there is a step regarding c~t. Use tanh function to create candidate vectors, then use the result of point-wise multiplication of it and c~t, with ct updated as in Eq. (4). Current information is added by deleted information. The final step is to determine the content of output. In Eq. (5), xt and ht−1 as σ(⋅) function parameters are used to obtain ot, which performs point-wise multiplication with tanh(ct) to obtain ht. With continuous iteration of the algorithm, the simple gradient descent enables weight W,b to gradually approach the optimal solution, after training is completed. Calculated ht is another result we want to predict.

Figure 3: LSTM model architecture

2.3.3 D-LSTM Design

So that data TA indicators would have a better effect on the actual network, we designed the D-LSTM model shown in Fig. 4. As described in Section 2.1, indicators include data Open, Close, High, Low, Volume, and Turnover. The data of the first four groups and transaction volume are of different orders of magnitude. Previous data should be normalized as input to the network before training. As shown in Eq. (6), where x represents the original value, after normalization, all data is compressed into a [−1,1] interval. This normalization method can not only reduce the difference in magnitude of the data, which may disable gradient decline, but also increase convergence speed and prediction accuracy [18]. As shown in Fig. 4, the increasing number of FC layers helps improve the network’s ability to predict nonlinearity. The output of this network will be used as a member of the final network, which is combined with news N-CNN network for the final calculation. Normalized data xS is pooled through one FC layer and the LSTM layer, and then through the tanh function layer and finally the FC layer to generate the final predicted value. The loss is calculated as the difference between the normalized predicted value and the actual value. It is worth noting that we spend enough time to train all 1014 stocks in the TA indicator dataset, and make sure that the D-LSTM model is stable enough to be used in the final CNN-LSTM model.

x∗=x−xmeanxmax−xmin (6)

Figure 4: D-LSTM network

2.3.4 CNN-LSTM Design

The CNN is designed to verify the effectiveness of the N-CNN network. When using the network, D-LSTM is trained using data xS, and N-CNN is trained with data xN. They are both input into CNN-LSTM. Final text data xF is different from the time period obtained by xS and xN. Dataset xF contains indicators of D-LSTM and includes news headlines text of the N-CNN network, as shown in Fig. 5.

Figure 5: CNN-LSTM model architecture

(1)   The CNN network results predict the value of the stock’s rise and fall on next day from news headline marked as [O1, O2]. O1 shows probability of the stock’s price falling, and O2 shows its probability of rising. Due to the property of the softmax function, the two probabilities O1 + O2 = 1. Using O2 as the final compensation formula, if O2>0.5, it means the next day data HIGH will be rising; if O2<0.5, it means it will go down. In CNN-LSTM net O2 rewrite by Ye as a parameter of final compensation formula.

(2)   The result of LSTM Yp is the prediction calculated from the trained network D-LSTM based on TA indicator part of xF, Yp give a specific price next day. Stock price trend is mainly controlled by shareholders, and shareholders will obviously influenced by news. When the stock news forecast’s probability of rise and fall is accurate enough to predict the trend of the stock price, then the correct value of the final algorithm is obtained by adding the compensation value calculated by O2 on basis of Yp, so that ordinary LSTM can further reduce the loss.

(3)   Compensation parameter ∂

Based on the D-LSTM model, calculating the highest price of stock on the second day is done by the test set. When set to Yt, there is

Ω=1n∑Yt−Yp (7)

N is a number of test sets per stock, and Ω is called predictive absolute error. The final predicted value Yfi is calculated by compensation Eq. (8), where ∂ is the weight of the compensation formula. Because only one xF set of datasets used to verify the final prediction result is not enough for network training to obtain the optimal solution, we grouped xF manually in experimental part and set ∂ to different values in order to select the best result.

Yfi=Yp+∂(Ye−0.5)∗Ω (8)

3  Experiment

This section discusses the actual situation of N-CNN network accuracy using horizontally extended news headline data, D-LSTM, and the test results of CNN-LSTM from final data xF.

3.1 Results of N-CNN

The training set uses cross-entropy loss function. The formula for calculating this is shown in (9). Parameter p indicates the distribution of true tokens, that is, the real price of the label layer on the next day. Parameter q is the predicted distribution of the training model, and it reflects the similarity between the predicted value of text and the actual value. The lower the loss value, the closer the training set to real data. This training dataset model used in this paper results in highly consistent accuracy under different cycle times. However, in this experiment, training set accuracy is a less important criterion than test set accuracy, which is used as the final evaluation standard for proving the generalization ability of CNN in natural language processing (NLP).

H(p,q)=∑xp(x)⋅log⁡(1q(x)) (9)

If all data xNi (0<i≤k) in data xN1, xN2,…, xNk (k = 1014) have lx (1≤x≤30) news headlines, then lx news is divided into words and converted into input word vectors that are accepted by the CNN. For the initial parameter settings of the N-CNN model, the number of convolutional layer filters is 128, and the fully connected layer is 96 neurons. As can be seen in Tab. 2, the experimental results show that cross-entropy of the training set decreases with the increase in training times, which indicates that the performance of the training set is gradually improved, but the constantly increasing accuracy of the test set means a decline in generalization ability. The best cycle time for the N-CNN algorithm is 200, and the test set has the highest accuracy. Therefore, in all subsequent experiments, the number of training cycles of the N-CNN algorithm was 200, and its accuracy reached as high as 73.40%. For an algorithm that tracked only one stock and had an actual sample size of about 100, accuracy was less than 56%. This meant it was feasible to increase the sample size without taking into consideration the sensitivity of different stock news. Algorithms used to predict stock market trends need to have an accuracy of more than 50%. This way, shareholders can rely on news to buy or sell stocks, rather than trying to find useful information from complex data indicators.

The following experiment was used to verify the impact of data sample size on N-CNN. We divided the 1014 stocks into 20%, 40%, 60%, 80%, and 100% samples, and kept the training set and test set ratio at 8:2. The input news text was also reduced accordingly. The most effective of all other parameters from the previous experiment, including training cycle of the training set, were the same. These experimental results are shown in Tab. 3 and Fig. 6.

Figure 6: Relationship between sample size and accuracy

It can be seen that when data volume was below the 30,000 level, sample size is proportional to accuracy. This means that as long as the number of samples obtained is increased, accuracy will inevitably continue to increase until it reaches a level of saturation. This means that theoretically, if the amount of text for all news headlines for stocks listed on the China stock exchange, that is, a level of 10 million, then accuracy will increase significantly.

The other algorithm models used for forecasting stock prices are shown in Tab. 4. The Ding and Wang Y algorithms use the time series of several stocks, which results in different accuracy between different stock data samples. The algorithm proposed in this paper uses the average of all 1014 stocks.

3.2 Results of D-LSTM

The results of D-LSTM are similar to forecasting the trend of the next day’s stock market with that day’s basic indicator. The most commonly used equations for evaluating an algorithm are Eqs. (10) to (12)

RMSE=1N∑i=1N(x1,i−x2,i)2 (10)

RMAE=1N∑i=1N∣x1,i−x2,i| (11)

MAPE=100N∑i=1N|x2,i−x1,ix1,i| (12)

This paper uses root-mean-square-error (RMSE) as the loss function for training of the training set in the D-LSTM, because RMSE is a good indicator for verifying the sample prediction error [9]. After training with RMSE loss function, we verified the prediction results of the other two indicators for the test set, and used root-mean-absolute-error (RMAE) to verify the deviation of the model system. In Eqs. (10) to (12), N is the number of samples to be calculated, x1,i is the predicted value, and x2,i is the actual value.

In the D-LSTM model, we set the number of trainings to 200, and the number of neurons in the FC and LSTM layers as 256. In the final CNN-LSTM model, the D-LSTM part of the experimental data used basic indicators of 600718–603993. The results are randomly selected and shown in Tab. 5.

The Shanghai Composite Index has the largest amount of data and seems to be the most stable. The remaining stocks are all individual stocks. Some effects in the D-LSTM algorithm are worse than SSE, and some are better than SSE. The last line shows the average of the results of all the experiments. This average shows that individual stocks and the SSE have similar effects using D-LSTM model, and can reduce the RMSE to a level below 0.02.

3.3 Results of CNN-LSTM

Once we determined that news headlines did play a role in forecasting stock prices on the second day of the stock market, and the accuracy rate was greater than 70%, then we discovered that there was a way to further increase the accuracy of forecast value. An experiment was carried out by the steps described in CNN-LSTM design part. The time of final data xF is independent of news data and TA data, which can to a great extent verify the final results of the experiment. The results of this experiment are shown is Tab. 6.

The experimental results show that the values of RMSE, RMAE, and mean absolute percentage error (MAPE) are the smallest when ∂ is set to 1.5. The result of ∂ =1.5 is compared with the original D-LSTM experiment, where RMSE, RMAE, and MAPE increased by about 8.3%, about 4%, and about 36.6%, respectively.

4  Conclusion

This paper proposed a model for looking at an increased volume of data, consisting of numerous stock news headlines, to improve the accuracy of CNN. Our proposed N-CNN model calculates the difference between predicted stock price values and actual values through the use of a compensation formula, which further improves the results from LSTM in stock price forecasting.

There are still some things we need to do in the future. First, news headlines lack a standardized time format. Since stock-related news is not subject to standardized regulation, several articles are produced each day, with some stocks having three articles on the same day and some having only one article. As a result, N-CNN input dimensions are not unified, which result in a flaw in training of the CNN model, which in turn reduces the accuracy of the test set. Second, when news headline text is converted into a word vector, numbers will be filtered out. This aspect can successfully filter out information of stock search index that often appears in a news headline, so that the model can focus on information and filter out the value of financial or stock price data, which can improve the accuracy of forecast results. Third, different stocks have different sensitivities to news. If the sensitivity of each stock to news is considered in the N-CNN model, deep learning gives each stock different sensitivity, which will also improve the results.

Acknowledgement: We thank LetPub (https://www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

Funding Statement: This work was supported by the Natural Science Foundation of China (61572160).

Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.

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