## Non-Negative Matrix Factorization and Term Structure of Interest Rates

21/09/2014**Auteurs :**

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Non-Negative Matrix Factorization and Term Structure of Interest Rates Hellinton H. Takadaa, b and Julio M. Sternb a Quantitative Research, Itaú Asset Management, São Paulo, Brazil b Institute of Mathematics and Statiscs, University of São Paulo, São Paulo, Brazil Abstract. Non-Negative Matrix Factorization (NNMF) is a technique for dimensionality reduction with a wide variety of applications from text mining to identification of concentrations in chemistry. NNMF deals with non-negative data and results in non-negative factors and factor loadings. Consequently, it is a natural choice when studying the term structure of interest rates. In this paper, NNMF is applied to obtain factors from the term structure of interest rates and the procedure is compared with other very popular techniques: principal component analysis and Nelson-Siegel model. The NNMF approximation for the term structure of interest rates is better in terms of fitting. From a practitioner point of view, the NNMF factors and factor loadings obtained possess straightforward financial interpretations due to their non- negativeness. Keywords: Information theory, Entropy, Financial markets. PACS: 89.70.-a, 89.70.Cf, 89.65.Gh INTRODUCTION Non-Negative Matrix Factorization (NNMF) is a multivariate data analysis technique aimed to estimate non- negative factors and factor loadings from non-negative data. NNMF was invented by Paatero and Tapper in 1994 under the name Positive Matrix Factorization (PMF) [1] and the name NNMF was established by Lee and Seung in 1999 [2]. There are several applications of NNMF and some examples from the literature are: text mining [3], image processing [4], sound processing [5], identification of concentrations in chemistry [1], recognition of underlying trends in stock market data [6], and so on. There are many algorithms for NNMF with different optimization strategies such as multiplicative update [4], gradient descent [4] or alternating least squares [1]. The term structure of interest rates or the yield curve is the relationship between interest rates or bond yields and different maturities or terms. The yield curve is important in economy and finance because it reflects current expectations of market participants about future changes in the interest rates. There are several factor models for the yield curve: Litterman and Sheinkman (1991) proposed a three factor model based on Principal Component Analysis (PCA) and suggested names for these factors: level, steepness (or slope) and curvature [7]. Since then, these factors became attributes of the yield curve. Independently, Nelson and Siegel (1987) published a parametric model for the yield curve [8] which was rewritten by Diebold and Li (2006) in terms of the yield curve attributes [9]. A plethora of models evolved from these two approaches. Obviously, the yield curve data is non-negative by nature and, consequently, it is appropriate to use NNMF. Since a model for yield curve has the objective to give insights about the current expectations about the future for economic and financial analysis, the non-negativity of factors and factor loadings is desired to easy the interpretation. Given a number of factors, PCA does not minimize the approximation error. Additionally, Nelson- Siegel (NS) model presents some fitting problems and a fourth factor was included by Svensson (1994) to improve the original model [10]. Fixing the number of factors, NNMF algorithms reduce the approximation error of the factorization. Consequently, it is expected a better fitting for NNMF when compared with PCA or NS. There is a large variety of divergence measures which have been applied as objective functions when minimizing the approximation error of NNMF: Bregman divergences [11], Renyi's information measure [12], Csiszar's divergences [13], Kompass' divergence [14], the -divergence [15] or Itakura-Saito divergence [16]. The paper is organized as follows: Firstly, the PCA approach proposed by Litterman and Sheinkman (1991) and the NS parametric factor model are presented. Then, the NNMF problem is reviewed with some details. After the theory, some results using real yield curve data are presented and the obtained factors and factor loadings are compared to that from PCA and NS. Finally, the conclusion together with more comments about the results are given at the end. FACTORS AND TERM STRUCTURE OF INTEREST RATES Principal Component Analysis Since the three factor model from Litterman and Sheinkman (1991), PCA has been used extensively to model the term structure of interest rates and it was verified that a large portion of bond return variation (up to 98%) can be explained by the first three principal components or factors: level, steepness and curvature. PCA is a statistical technique to convert a set of observations of possibly correlated variables into a set of values of linearly uncorrelated variables called principal components using orthogonal transformation [17]. Singular Value Decomposition (SVD) is technique from linear algebra used to obtain the principal components [18]. Given a matrix of historical yields with days and vertices or maturities, the SVD factorization results: (1) where is obtained mean centering the data matrix , , , , , , the columns of and are orthonormal eigenvectors of and is a diagonal matrix containing the square roots of the corresponding eigenvalues from or such that , since usually . Given , the -factor model using PCA is given by: (2) where and . The columns of are called factors and the columns of are the corresponding factor loadings. When , the first column of is named level, the second is the steepness and the third is the curvature. Nelson-Siegel Model The Nelson and Siegel proposed a parametric model in 1987 and it has been used to fit the term structure of interest rates. Using the formulation from [9], the Nelson-Siegel (NS) is given by: (3) where is the maturity (usually in years), is the curve yield at maturity and , and are cross- sectional parameters to be determined for each date . It is straightforward to notice that these parameters can be determined via a least-squares or similar algorithm [9,20]. The parameter can be determined to minimize the possible correlation of and over the time to avoid possible identification problems. There are important interpretations for some of the parameters: represents the long run level of interest rates, is short-term interest rate and is the medium term component. Using the names from Litterman and Sheinkman (1991), represents the level, captures the steepness and is the curvature. Non-Negative Matrix Factorization NNMF seems to be suitable to model the factors behind the term structure of interest rates. Since the matrix of historical yields is nonnegative and given an integer , the NNMF problem is to find the following approximation: (4) where , and . It is possible to notice that represents the factors and the factor loadings. The NNMF optimization procedures minimizes the approximation error between and . In a generalized way, the Bregman divergence is used as the objective function to be minimized [11,19]. Considering only separable Bregman divergences, (5) where is a strictly convex function with a continuous first derivative. Formally, the resulting optimization problems are: (6) or (7) where and are penalty functions to enforce certain application-dependent characteristics of the solution, such as sparsity and/or smoothness. It is also important to remember that the Bregman divergences are not symmetric in general. Consequently, it will be considered here . Adopting and , there are some known algorithms to solve the NNMF problem divided in general classes [21]: gradient descent algorithms, multiplicative update algorithms and alternating least squares algorithms (ALS). Actually, when the Bregman divergence is the squared Frobenius norm. For example, when the Bregman divergence is the Kullback-Leibler divergence. EMPIRICAL COMPARISON OF FACTORS In this section, the objective is to present and compare the factors and factor loading obtained from real yield curve data using PCA, NS and NNMF. The data used for the study is the Brazilian term structure of interest rates obtained from future contracts traded at BM&FBovespa. It was used data from 05/13/2003 to 10/09/2013. The vertices chosen are: 3 months, 6 months, 1 year, 2 years and 5 years. To be able to compare the different approaches and having in mind the factors named by Litterman and Sheinkman (1991), the number of factors considered here for the models under comparison is three. The PCA was implemented using SVD, NS was obtained using an optimization procedure to minimize the least- square error and NNMF was calculated using ALS with multiple starting points. FIGURE 1 presents the PCA factors and the corresponding factor loading are in FIGURE 2. Observing the PCA factor loadings, it becomes clear the origin of the names given by Litterman and Sheinkman (1991) for each factor. The obtained NS factors and factor loadings are in FIGURE 3 and FIGURE 4, respectively. The NS factor loadings also capture the idea of level, steepness and curvature. As already mentioned, the factors are estimated for each day in the sample. Comparing with the PCA factors, the NS factors over the time are not smooth with several spikes and, consequently, difficult for economic or financial interpretation. FIGURE 1. Factors for yield curve using PCA. FIGURE 2. Factor loadings for yield curve using PCA. FIGURE 3. Factors for yield using NS. FIGURE 4. Factor loadings for yield curve using NS. The factors and factor loadings from NNMF are in FIGURE 5 and FIGURE 6, respectively. The obtained factor loadings are not the same ones pointed by Litterman and Sheinkman (1991). Actually, it is possible to identify a steepness factor and two different curvatures. The inclusion of a second curvature to model the yield curve is not new in the literature. Actually, a fourth factor is usually included in the NS framework under the name Svensson's factor to improve the data adjustment [10]. The Nelson-Siegel-Svensson model is given by: (8) where is the fourth factor representing a second curvature, and represent the two curvatures and the remaining parameters are the same from . Clearly, the second curvature from NNMF brings the idea of the Svensson's factor. The NNMF obtained factors are smooth and, consequently, it is natural to use them for investment strategies or economic/financial analysis about the future behavior of the term structure of interest rates related to its steepness and two curvatures. The non-negativeness of the factors and factor loadings implies in a easy interpretation: when a factor increases, the yield curve explanation by the corresponding factor loading also increases and vice versa. Finally, the approximation error for each factor model using the Frobenius norm is in TABLE 1. Since the NNMF minimizes the approximation error, the data fitting is better. Obviously, it is not the purpose of PCA to improve the fitting given a number a factors and the NS is not being able to adjust perfectly its parametric form to the data. FIGURE 5. Factors for yield curve using NNMF. FIGURE 6. Factor loadings for yield curve using NNMF. TABLE 1. Approximation errors for PCA, NS and NNMF. Factor Models Approximation Error PCA NS NNMF CONCLUSIONS In this paper, NNMF technique was applied for the first time to find factors for yield curves. The data adjustment is far better than that obtained from usual techniques such as PCA or NS. The obtained factors and factor loadings due to their non-negativeness easy the interpretation for economic and financial analysis. Additionally, the NNMF factors are relatively smooth when compared to that from NS and they are quite different from the classical ones from Litterman and Sheinkman (1991). Actually, for the Brazilian data the three NNMF factors are identified as the steepness and two different curvatures. One of the curvatures is that one identified by Litterman and Sheinkman (1991) and the other is the called Svensson's factor. In this work, NNMF was found to be a suitable factorization model for yield curves. REFERENCES 1. P. Paatero and U. Tapper, "Positive matrix factorization: a non-negative factor model with optimal utilization of error estimates of data values," Environmetrics 5 (2), 111–126 (1994). 2. D. Lee and H. Seung, "Learning the parts of objects by non-negative matrix factorization," Nature 401, 788–791 (1999). 3. M. W. Berry (editor), Computational information retrieval, Philadelphia: Society for Industrial and Applied Mathematics, 2001. 4. D. Lee and H. 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