jmlr jmlr2010 jmlr2010-110 knowledge-graph by maker-knowledge-mining

110 jmlr-2010-The SHOGUN Machine Learning Toolbox


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Author: Sören Sonnenburg, Gunnar Rätsch, Sebastian Henschel, Christian Widmer, Jonas Behr, Alexander Zien, Fabio de Bona, Alexander Binder, Christian Gehl, Vojtěch Franc

Abstract: We have developed a machine learning toolbox, called SHOGUN, which is designed for unified large-scale learning for a broad range of feature types and learning settings. It offers a considerable number of machine learning models such as support vector machines, hidden Markov models, multiple kernel learning, linear discriminant analysis, and more. Most of the specific algorithms are able to deal with several different data classes. We have used this toolbox in several applications from computational biology, some of them coming with no less than 50 million training examples and others with 7 billion test examples. With more than a thousand installations worldwide, SHOGUN is already widely adopted in the machine learning community and beyond. SHOGUN is , implemented in C++ and interfaces to MATLABTM R, Octave, Python, and has a stand-alone command line interface. The source code is freely available under the GNU General Public License, Version 3 at http://www.shogun-toolbox.org. Keywords: support vector machines, kernels, large-scale learning, Python, Octave, R

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 Journal of Machine Learning Research 11 (2010) 1799-1802 Submitted 10/09; Revised 3/10; Published 6/10 The SHOGUN Machine Learning Toolbox S¨ ren Sonnenburg∗ o SOEREN . [sent-1, score-0.044]

2 28/29, 10587 Berlin, Germany Gunnar R¨ tsch a Sebastian Henschel Christian Widmer Jonas Behr Alexander Zien† Fabio de Bona GUNNAR . [sent-4, score-0.103]

3 28/29, 10587 Berlin, Germany Vojtˇ ch Franc e XFRANCV @ CMP. [sent-26, score-0.036]

4 It offers a considerable number of machine learning models such as support vector machines, hidden Markov models, multiple kernel learning, linear discriminant analysis, and more. [sent-30, score-0.095]

5 We have used this toolbox in several applications from computational biology, some of them coming with no less than 50 million training examples and others with 7 billion test examples. [sent-32, score-0.154]

6 With more than a thousand installations worldwide, SHOGUN is already widely adopted in the machine learning community and beyond. [sent-33, score-0.101]

7 SHOGUN is , implemented in C++ and interfaces to MATLABTM R, Octave, Python, and has a stand-alone command line interface. [sent-34, score-0.114]

8 The source code is freely available under the GNU General Public License, Version 3 at http://www. [sent-35, score-0.031]

9 Introduction With the great advancements of machine learning in the past few years, many new learning algorithms have been proposed, implemented in C/C++, and made publicly available. [sent-39, score-0.067]

10 However, for example, there currently exist more than 20 different publicly available implementations of Support Vector Machine (SVM) solvers. [sent-51, score-0.129]

11 Each one comes with its own interface, a small set of available kernel functions, and unique benefits and drawbacks. [sent-52, score-0.063]

12 There is no single unified way of interfacing with these implementations, even though they all are based on essentially the same methodology of supervised learning. [sent-53, score-0.028]

13 This restraints users from fully taking advantage of the recent developments in machine learning algorithms. [sent-54, score-0.03]

14 This motivated us to develop a machine learning toolbox that provides an easy, unified way for solving certain types of machine learning problems. [sent-55, score-0.078]

15 The result is a toolbox, called SHOGUN, with a focus on large-scale learning using kernel methods and SVMs. [sent-56, score-0.063]

16 It provides a generic interface to 15 SVM implementations, among them SVMlight, LibSVM, GPDT, SVMLin, LibLinear, and OCAS. [sent-57, score-0.115]

17 The SVMs can be easily combined with more than 35 different kernel functions. [sent-58, score-0.063]

18 Moreover, it offers options for using precomputed kernels and allows easy integration of new implementations of kernels. [sent-63, score-0.177]

19 One of SHOGUN’s key features is the combined kernel to construct weighted linear combinations of multiple kernels that may even be defined on different input domains. [sent-64, score-0.149]

20 Also, several Multiple Kernel Learning algorithms based on different regularization strategies are available to optimize the weighting of the kernels (e. [sent-65, score-0.086]

21 In addition to kernel and distance based methods, SHOGUN implements many linear methods and features algorithms to train hidden Markov models. [sent-71, score-0.095]

22 R¨ tsch and Sonnenburg, 2007; a Schweikert et al. [sent-73, score-0.103]

23 The input feature objects can be dense or sparse vectors of strings, integers (8, 16, 32 or 64 bit; signed or unsigned), or floating point numbers (32 or 64 bit), and can be converted into different feature types. [sent-75, score-0.034]

24 Finally, several commonly used performance measures, like accuracy and area under ROC or precision-recall curves, are implemented in SHOGUN. [sent-79, score-0.035]

25 Moreover, whenever possible, we implemented auxiliary routines that allow faster computation of combinations of kernel elements that lead to significant speedup during training (for some SVM implementations, e. [sent-82, score-0.098]

26 This allowed us to use SHOGUN for solving several large-scale learning problems in biological sequence analysis, for example, splice site recognition with up to 50 million example sequences for training (Sonnenburg et al. [sent-87, score-0.076]

27 , 2007; Franc and Sonnenburg, 2009) and transcription start site recog1. [sent-88, score-0.095]

28 Complete lists of SVM and kernel implementations together with user and developer documentation is available at http://www. [sent-89, score-0.217]

29 1800 T HE SHOGUN M ACHINE L EARNING T OOLBOX nition with almost 7 billion test sequences (Sonnenburg et al. [sent-92, score-0.044]

30 SHOGUN’s core functions are encapsulated in a library (libshogun) and are easily accessible and extendible by C++ application developers. [sent-94, score-0.105]

31 What sets SHOGUN apart from many other machine learning toolboxes, is that it provides interactive user interfaces to most major scripting languages that are currently used in scientific computing, in particular Python, MATLAB, Octave, R, and a command-line version. [sent-95, score-0.26]

32 All classes and functions are documented and come with over 600 examples and a tutorial for new users and developers is part of the release. [sent-96, score-0.03]

33 Maintaining high code quality is ensured by a test suite that supports running the algorithms for each interface on predefined inputs in order to detect breakage. [sent-102, score-0.146]

34 Modular, Extendible Object-Oriented Design SHOGUN is implemented in an object-oriented way using C++ as the programming language. [sent-105, score-0.035]

35 All objects inherit from CSGObject, which provides means for garbage collection via reference counting, serialization, and versioning of the object. [sent-106, score-0.062]

36 The implementations of many classes employ templates enabling SHOGUN’s support of many different data types without code duplication. [sent-107, score-0.127]

37 As the source code and user documentation is automatically generated using doxygen and written in-place in the header files, it also drastically reduces the amount of work needed to maintain the documentation. [sent-108, score-0.15]

38 As an example of SHOGUNs object-oriented design, consider the class CClassifier: From this class, CKernelMachine, for example, is derived and provides basic functions for applying a trained kernel classifier (computing f (x) = ∑N αi k(x, xi ) + b) thus enabling code re-use wheni=1 ever possible. [sent-109, score-0.127]

39 Interfaces to Scripting Languages and Applications Built around SHOGUN’s core are two types of interfaces: A modular interface that makes use of the SWIG (http://www. [sent-116, score-0.198]

40 Thanks to SWIG, the modular interface provides the exact same objects in a modular object-oriented way that are available from C++ to other languages, such as R, Python, and Octave. [sent-119, score-0.315]

41 Using so-called typemaps, it is convenient to provide type mappings from the native datatype used in the interface to SHOGUN. [sent-120, score-0.115]

42 For example, a function void set features(double* features, int n, int m) can be called directly from Octave with a single matrix argument, for example, set features(randn(3,4)). [sent-121, score-0.072]

43 The variables n and m are then automatically set to the matrix dimensions and together with a data pointer passed to the SHOGUN core. [sent-122, score-0.028]

44 SHOGUN also provides a static interface with the same structure for all supported plattforms , including the command-line interface where inputs are provided as either strings or files. [sent-123, score-0.307]

45 1801 ¨ S ONNENBURG , R ATSCH , H ENSCHEL , W IDMER , B EHR , Z IEN , DE B ONA , G EHL , B INDER AND F RANC implemented independent of the target language through the class CSGInterface, which provides abstract functions to deliver or obtain data from any particular plattform. [sent-134, score-0.035]

46 A community around SHOGUN is continuously developing, with a growing number of projects building on it (cf. [sent-135, score-0.035]

47 org/software/tags/shogun) and a mailing list with more than 100 subscribed users. [sent-137, score-0.028]

48 4 By 10/2009, there had been at least 1, 100 installations under the Linux distributions Debian and Ubuntu. [sent-138, score-0.066]

49 We will continue to develop SHOGUN and are confident that it is and will continue to be useful, and will make an increasing impact beyond the machine learning community by benefiting diverse applications. [sent-139, score-0.035]

50 Support vector machines and o a kernels for computational biology. [sent-143, score-0.086]

51 Efficient and accurate u l p -norm multiple kernel learning. [sent-149, score-0.063]


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Again, the interface is simple since only the I ∂ evaluation of the covariance matrix K = kψ (X) and its derivatives ∂i K = ∂ψi kψ (X) on a data set X are required. Furthermore, we need cross terms k∗ = kψ (X, x∗ ) and k∗∗ = kψ (x∗ , x∗ ) for prediction. There are no restrictions on the composition of both mean and covariance functions—any combination is allowed including nested composition. 3013 R ASMUSSEN AND N ICKISCH √ √ The Laplace (lik/likLaplace.m) likelihood Pρ (y| f ) = exp(− 2/σn |y − f |)/ 2σn with hyperparameters ρ = {ln σn } is specified in line 3. There are only simple likelihood functions: Gaussian, Sech-squared, Laplacian and Student’s t for ordinary and sparse regression as well as the error and the logistic function for classification. Again, the same inference code is used for any likelihood function. Although the specification of likelihood functions is simple for the user, writing new likelihood functions is slightly more involved as different inference methods require access to different properties; for example, LA requires second derivatives and EP requires derivatives of moments. All hyperparameters θ = {φ, ψ, ρ} are stored in a struct hyp.{mean,cov,lik}, which is initialised in line 4; we select the approximate inference algorithm EP (inf/infEP.m) in line 5. We optimise the hyperparameters θ ≡ hyp by calling the CG optimiser (util/minimize.m) with initial value θ0 ≡ hyp0 in line 6 allowing at most N = 50 evaluations of the EP approximation to the marginal likelihood ZEP (θ) as done by gp.m. Here, D = (X, y) ≡ (X,y) is the training data where X = {x1 , .., xn } and y ∈ Rn . Under the hood, gp.m computes in every step a Gaussian ∂ posterior approximation and the derivatives ∂θ ln ZEP (θ) of the marginal likelihood by calling EP. Predictions with optimised hyperparameters are done in line 7, where we call gp.m with the unseen test inputs X∗ ≡ Xs as additional argument. As a result, we obtain the approximate marginal predictive mean E[P(y∗ |D , X∗ )] ≡ ymu and the predictive variance V[P(y∗ |D , X∗ )] ≡ ys2. Likelihood \ Inference Gaussian Sech-squared Laplacian Student’s t Error function Logistic function Exact FITC EP Laplace VB Type, Output Domain regression, R regression, R regression, R regression, R classification, {±1} classification, {±1} Alternate Name logistic distribution double exponential probit regression logit regression Table 1: Likelihood ↔ inference compatibility in the GPML toolbox Table 1 gives the legal likelihood/inference combinations. Exact inference and the FITC approximation support the Gaussian likelihood only. Variational Bayesian (VB) inference is applicable to all likelihoods. Expectation propagation (EP) for the Student’s t likelihood is inherently unstable due to its non-log-concavity. The Laplace approximation (LA) for Laplace likelihoods is not sensible due to the non-differentiable peak of the Laplace likelihood. Special care has been taken for the non-convex optimisation problem imposed by the combination Student’s t likelihood and LA. If the number of training examples is larger than a few thousand, dense matrix computations become too slow. We provide the FITC approximation for regression with Gaussian likelihood where ˜ instead of the exact covariance matrix K, a low-rank plus diagonal matrix K = Q + diag(K − Q) ⊤ K−1 K is used. The matrices K and K contain covariances and cross-covariances where Q = Ku uu u uu u of and between inducing inputs ui and data points x j . Using inf/infFITC.m together with any covariance function wrapped into cov/covFITC.m makes the computations feasible for large n. Acknowledgments Thanks to Ed Snelson for assisting with the FITC approximation. 3014 G AUSSIAN P ROCESSES FOR M ACHINE L EARNING T OOLBOX References Joaquin Qui˜ onero Candela and Carl E. Rasmussen. A unifying view of sparse approximate Gausn sian process regression. Journal of Machine Learning Research, 6(6):1935–1959, 2005. Mark N. Gibbs and David J. C. MacKay. Variational Gaussian process classifiers. IEEE Transactions on Neural Networks, 11(6):1458–1464, 2000. Thomas P. Minka. Expectation propagation for approximate Bayesian inference. In UAI, pages 362–369. Morgan Kaufmann, 2001. Hannes Nickisch and Carl E. Rasmussen. Approximations for binary Gaussian process classification. Journal of Machine Learning Research, 9:2035–2078, 10 2008. Carl E. Rasmussen and Christopher K. I. Williams. Gaussian Processes for Machine Learning. The MIT Press, Cambridge, MA, 2006. Ed Snelson and Zoubin Ghahramani. Sparse Gaussian processes using pseudo-inputs. In Advances in Neural Information Processing Systems 18, 2006. Christopher K. I. Williams and D. Barber. Bayesian classification with Gaussian processes. IEEE Transactions on Pattern Analysis and Machine Intelligence, 12(20):1342–1351, 1998. 3015

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Unfortunately, while allowing scientists more flexibility to study phenomena under controlled conditions, computer simulations require a substantial investment of c 2010 Dirk Gorissen, Ivo Couckuyt, Piet Demeester, Tom Dhaene and Karel Crombecq. G ORISSEN , C OUCKUYT, D EMEESTER , D HAENE AND C ROMBECQ computation time. One simulation may take many minutes, hours, days or even weeks, quickly rendering parameter studies impractical (Forrester et al., 2008; Simpson et al., 2008). Of the different ways to deal with this problem, this paper is concerned with the construction of simpler approximation models to predict the system performance and develop a relationship between the system inputs and outputs. When properly constructed, these approximation models mimic the behavior of the simulation accurately while being computationally cheap(er) to evaluate. Different approximation methods exist, each with their relative merits. This work concentrates on the use of data-driven, global approximations using compact surrogate models (also known as metamodels, replacement models, or response surface models). Examples include: rational functions, Kriging models, Artificial Neural Networks (ANN), splines, and Support Vector Machines (SVM). Once such a global approximation is available it is of great use for gaining insight into the behavior of the underlying system. The surrogate may be easily queried, optimized, visualized, and seamlessly integrated into CAD/CAE software packages. The challenge is thus how to generate an approximation model that is as accurate as possible over the complete domain of interest while minimizing the simulation cost. Solving this challenge involves multiple sub-problems that must be addressed: how to interface with the simulation code, how to run simulations (locally, or on a cluster or cloud), which model type to approximate the data with and how to set the model complexity (e.g., topology of a neural network), how to estimate the model quality and ensure the domain expert trusts the model, how to decide which simulations to run (data collection), etc. The data collection aspect is worth emphasizing. Since data is computationally expensive to obtain and the optimal data distribution is not known up front, data points should be selected iteratively, there where the information gain will be the greatest. A sampling function is needed that minimizes the number of sample points selected in each iteration, yet maximizes the information gain of each iteration step. This process is called adaptive sampling but is also known as active learning, or sequential design. There is a complex dependency web between these different options and dealing with these dependencies is non-trivial, particularly for a domain expert for whom the surrogate model is just an intermediate step towards solving a larger, more important problem. Few domain experts will be experts in the intricacies of efficient sampling and modeling strategies. Their primary concern is obtaining an accurate replacement metamodel for their problem as fast as possible and with minimal overhead (Gorissen et al., 2009d). As a result these choices are often made in a pragmatic, sometimes even ad-hoc, manner. This paper discusses an advanced, and integrated software framework that provides a flexible and rigorous means to tackle such problems. This work lies at the intersection of Machine Learning/AI, Modeling and Simulation, and Distributed Computing. The methods developed are applicable to any domain where a cheap, accurate, approximation is needed to replace some expensive reference model. Our experience has been that the availability of such a framework can facilitate the transfer of knowledge from surrogate modeling researchers and lower the barrier of entry for domain experts. 2. SUMO Toolbox The platform in question is the Matlab SUrrogate MOdeling (SUMO) Toolbox, illustrated in Figure 1. Given a simulation engine (Fluent, Cadence, Abaqus, HFSS, etc.) or other data source (data 2052 A S URROGATE M ODELING AND A DAPTIVE S AMPLING TOOLBOX FOR C OMPUTER BASED D ESIGN Figure 1: The SUMO Toolbox is a flexible framework for accurate global surrogate modeling and adaptive sampling (active learning). It features a rich set of plugins, is applicable to a wide range of domains, and can be applied in an autonomous, black-box fashion, or under full manual control. Written in Matlab and Java it is fully cross platform and comes with a large (60+) number of example problems. set, Matlab script, Java class, etc.), the toolbox drives the data source to produce a surrogate model within the time and accuracy constraints set by the user. The SUMO Toolbox adopts a microkernel design philosophy with many different plugins available for each of the different sub-problems:1 model types (rational functions, Kriging, splines, SVM, ANN, etc.), hyperparameter optimization algorithms (Particle Swarm Optimization, Efficient Global Optimization, simulated annealing, Genetic Algorithm, etc.), model selection algorithms (cross validation, AIC, Leave-out set, etc.), sample selection (random, error based, density based, hybrid, etc.), Design of Experiments (Latin hypercube, Box-Bhenken, etc.), and sample evaluation methods (local, on a cluster or grid). The behavior of each software component is configurable through a central XML file and components can easily be added, removed or replaced by custom implementations. In addition the toolbox provides ‘meta’ plugins. For example to automatically select the best model type for a given problem (Gorissen et al., 2009d) or to use multiple model selection or sample selection criteria in concert (Gorissen et al., 2010). Furthermore, there is built-in support for high performance computing. On the modeling side, the model generation process can take full advantage of multi-core CPUs and even of a complete cluster or grid. This can result in significant speedups for model types where the fitting process can be expensive (e.g., neural networks). Likewise, sample evaluation (simulation) can occur locally (with the option to take advantage of multi-core architectures) or on a separate compute cluster or grid (possibly accessed through a remote head-node). All interfacing with the grid middleware 1. The full list of plugins and features can be found at http://www.sumowiki.intec.ugent.be. 2053 G ORISSEN , C OUCKUYT, D EMEESTER , D HAENE AND C ROMBECQ (submission, job monitoring, rescheduling of failed/lost simulation points, etc.) is handled transparently and automatically (see Gorissen et al., 2009c for more details). Also, the sample evaluation component runs in parallel with the other components (non-blocking) and not sequentially. This allows for an optimal use of computational resources. In addition the SUMO Toolbox contains extensive logging and profiling capabilities so that the modeling process can easily be tracked and the modeling decisions understood. Once a final model has been generated, a GUI tool is available to visually explore the model (including derivatives and prediction uncertainty), assess its quality, and export it for use in other software tools. 3. Applications The SUMO Toolbox has already been applied successfully to a very wide range of applications, including RF circuit block modeling (Gorissen et al., 2009b), hydrological modeling (Couckuyt et al., 2009), Electronic Packaging (Zhu and Franzon, 2009), aerodynamic modeling (Gorissen et al., 2009a), process engineering (Stephens et al., 2009), and automotive data modeling (Gorissen et al., 2010). Besides global modeling capabilities, the SUMO Toolbox also includes a powerful optimization framework based on the Efficient Global Optimization framework developed by Jones et al. (1998). As of version 6.1, the toolbox also contains an example of how the framework can also be applied to solve classification problems. In sum, the goal of the toolbox is to fill the void in machine learning software when it comes to the challenging, costly, real-valued, problems faced in computational engineering. The toolbox is in use successfully at various institutions and we are continuously refining and extending the set of available plugins as the number of applications increase. Usage instructions, design documentation, and stable releases for all major platforms can be found at http://www.sumo.intec.ugent.be. References I. Couckuyt, D. Gorissen, H. Rouhani, E. Laermans, and T. Dhaene. Evolutionary regression modeling with active learning: An application to rainfall runoff modeling. In International Conference on Adaptive and Natural Computing Algorithms, volume LNCS 5495, pages 548–558, Sep. 2009. A. Forrester, A. Sobester, and A. Keane. Engineering Design Via Surrogate Modelling: A Practical Guide. Wiley, 2008. D. Gorissen, K. Crombecq, I. Couckuyt, and T. Dhaene. Foundations of Computational Intelligence, Volume 1: Learning and Approximation: Theoretical Foundations and Applications, volume 201, chapter Automatic approximation of expensive functions with active learning, pages 35–62. Springer Verlag, Series Studies in Computational Intelligence, 2009a. D. Gorissen, L. De Tommasi, K. Crombecq, and T. Dhaene. Sequential modeling of a low noise amplifier with neural networks and active learning. Neural Computing and Applications, 18(5): 485–494, Jun. 2009b. D. Gorissen, T. Dhaene, P. Demeester, and J. Broeckhove. Handbook of Research on Grid Technologies and Utility Computing: Concepts for Managing Large-Scale Applications, chapter Grid enabled surrogate modeling, pages 249–258. IGI Global, May 2009c. 2054 A S URROGATE M ODELING AND A DAPTIVE S AMPLING TOOLBOX FOR C OMPUTER BASED D ESIGN D. Gorissen, T. Dhaene, and F. DeTurck. Evolutionary model type selection for global surrogate modeling. Journal of Machine Learning Research, 10:2039–2078, 2009d. D. Gorissen, I. Couckuyt, E. Laermans, and T. Dhaene. Multiobjective global surrogate modeling,dealing with the 5-percent problem. Engineering with Computers, 26(1):81–89, Jan. 2010. D. R. Jones, M. Schonlau, and W. J. Welch. Efficient global optimization of expensive black-box functions. Journal of Global Optimization, 13(4):455–492, Nov. 1998. ISSN 0925-5001. T. W. Simpson, V. Toropov, V. Balabanov, and F. A. C. Viana. Design and analysis of computer experiments in multidisciplinary design optimization: a review of how far we have come or not. In Proceedings of the 12th AIAA/ISSMO Multidisciplinary Analysis and Optimization Conference, 2008 MAO, Victoria, Canada, 2008. D.W. Stephens, D. Gorissen, and T. Dhaene. Surrogate based sensitivity analysis of process equipment. In Proc. of 7th International Conference on CFD in the Minerals and Process Industries, CSIRO, Melbourne, Australia, Dec. 2009. T. Zhu and P. D. Franzon. Application of surrogate modeling to generate compact and PVT-sensitive IBIS models. In Proceedings of the 18th Conference on Electrical Performance of Electronic Packaging and Systems (EPEPS), Oct. 2009. 2055

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The order of components of the hyperparameters φ is the same as in the specification of the cell array. Every mean function implements its evaluation m = mφ (X) and first derivative ∂ computation mi = ∂φi mφ (X) on a data set X. In the same spirit, the squared exponential covariance kψ (x, x′ ) = σ f ² exp(− x − x′ 2 /2ℓ2 ) (cov/covSEiso.m) with hyperparameters ψ = {ln ℓ, ln σ f } is set up in line 2. Note, that the hyperparameters are represented by the logarithms, as these parameters are naturally positive. Many other simple covariance functions are contained in the toolbox. Among others, we offer linear, constant, Mat´ rn, rational quadratic, polynomial, periodic, neural network and finite support coe variance functions. Composite covariance functions allow for sums k(x, x′ ) = ∑ j k j (x, x′ ), products k(x, x′ ) = ∏ j k j (x, x′ ), positive scaling k(x, x′ ) = σ2 k0 (x, x′ ) and masking of components f k(x, x′ ) = k0 (xI , x′ ) with I ⊆ [1, 2, .., D], x ∈ RD . Again, the interface is simple since only the I ∂ evaluation of the covariance matrix K = kψ (X) and its derivatives ∂i K = ∂ψi kψ (X) on a data set X are required. Furthermore, we need cross terms k∗ = kψ (X, x∗ ) and k∗∗ = kψ (x∗ , x∗ ) for prediction. There are no restrictions on the composition of both mean and covariance functions—any combination is allowed including nested composition. 3013 R ASMUSSEN AND N ICKISCH √ √ The Laplace (lik/likLaplace.m) likelihood Pρ (y| f ) = exp(− 2/σn |y − f |)/ 2σn with hyperparameters ρ = {ln σn } is specified in line 3. There are only simple likelihood functions: Gaussian, Sech-squared, Laplacian and Student’s t for ordinary and sparse regression as well as the error and the logistic function for classification. Again, the same inference code is used for any likelihood function. Although the specification of likelihood functions is simple for the user, writing new likelihood functions is slightly more involved as different inference methods require access to different properties; for example, LA requires second derivatives and EP requires derivatives of moments. All hyperparameters θ = {φ, ψ, ρ} are stored in a struct hyp.{mean,cov,lik}, which is initialised in line 4; we select the approximate inference algorithm EP (inf/infEP.m) in line 5. We optimise the hyperparameters θ ≡ hyp by calling the CG optimiser (util/minimize.m) with initial value θ0 ≡ hyp0 in line 6 allowing at most N = 50 evaluations of the EP approximation to the marginal likelihood ZEP (θ) as done by gp.m. Here, D = (X, y) ≡ (X,y) is the training data where X = {x1 , .., xn } and y ∈ Rn . Under the hood, gp.m computes in every step a Gaussian ∂ posterior approximation and the derivatives ∂θ ln ZEP (θ) of the marginal likelihood by calling EP. Predictions with optimised hyperparameters are done in line 7, where we call gp.m with the unseen test inputs X∗ ≡ Xs as additional argument. As a result, we obtain the approximate marginal predictive mean E[P(y∗ |D , X∗ )] ≡ ymu and the predictive variance V[P(y∗ |D , X∗ )] ≡ ys2. Likelihood \ Inference Gaussian Sech-squared Laplacian Student’s t Error function Logistic function Exact FITC EP Laplace VB Type, Output Domain regression, R regression, R regression, R regression, R classification, {±1} classification, {±1} Alternate Name logistic distribution double exponential probit regression logit regression Table 1: Likelihood ↔ inference compatibility in the GPML toolbox Table 1 gives the legal likelihood/inference combinations. Exact inference and the FITC approximation support the Gaussian likelihood only. Variational Bayesian (VB) inference is applicable to all likelihoods. Expectation propagation (EP) for the Student’s t likelihood is inherently unstable due to its non-log-concavity. The Laplace approximation (LA) for Laplace likelihoods is not sensible due to the non-differentiable peak of the Laplace likelihood. Special care has been taken for the non-convex optimisation problem imposed by the combination Student’s t likelihood and LA. If the number of training examples is larger than a few thousand, dense matrix computations become too slow. We provide the FITC approximation for regression with Gaussian likelihood where ˜ instead of the exact covariance matrix K, a low-rank plus diagonal matrix K = Q + diag(K − Q) ⊤ K−1 K is used. The matrices K and K contain covariances and cross-covariances where Q = Ku uu u uu u of and between inducing inputs ui and data points x j . Using inf/infFITC.m together with any covariance function wrapped into cov/covFITC.m makes the computations feasible for large n. Acknowledgments Thanks to Ed Snelson for assisting with the FITC approximation. 3014 G AUSSIAN P ROCESSES FOR M ACHINE L EARNING T OOLBOX References Joaquin Qui˜ onero Candela and Carl E. Rasmussen. A unifying view of sparse approximate Gausn sian process regression. Journal of Machine Learning Research, 6(6):1935–1959, 2005. Mark N. Gibbs and David J. C. MacKay. Variational Gaussian process classifiers. IEEE Transactions on Neural Networks, 11(6):1458–1464, 2000. Thomas P. Minka. Expectation propagation for approximate Bayesian inference. In UAI, pages 362–369. Morgan Kaufmann, 2001. Hannes Nickisch and Carl E. Rasmussen. Approximations for binary Gaussian process classification. Journal of Machine Learning Research, 9:2035–2078, 10 2008. Carl E. Rasmussen and Christopher K. I. Williams. Gaussian Processes for Machine Learning. The MIT Press, Cambridge, MA, 2006. Ed Snelson and Zoubin Ghahramani. Sparse Gaussian processes using pseudo-inputs. In Advances in Neural Information Processing Systems 18, 2006. Christopher K. I. Williams and D. Barber. Bayesian classification with Gaussian processes. IEEE Transactions on Pattern Analysis and Machine Intelligence, 12(20):1342–1351, 1998. 3015

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