Unsupervised learning

Machine learning and data mining 

Machine learning venues

Unsupervised learning is the machine learning task of inferring a function to describe hidden structure from unlabeled data. Since the examples given to the learner are unlabeled, there is no error or reward signal to evaluate a potential solution. This distinguishes unsupervised learning from supervised learning and reinforcement learning.
Unsupervised learning is closely related to the problem of density estimation in statistics.^{[1]} However unsupervised learning also encompasses many other techniques that seek to summarize and explain key features of the data. Many methods employed in unsupervised learning are based on data mining methods used to preprocess^{[citation needed]} data.
Approaches to unsupervised learning include:
 clustering (e.g., kmeans, mixture models, hierarchical clustering),^{[2]}
 Approaches for learning latent variable models such as
Among neural network models, the selforganizing map (SOM) and adaptive resonance theory (ART) are commonly used unsupervised learning algorithms. The SOM is a topographic organization in which nearby locations in the map represent inputs with similar properties. The ART model allows the number of clusters to vary with problem size and lets the user control the degree of similarity between members of the same clusters by means of a userdefined constant called the vigilance parameter. ART networks are also used for many pattern recognition tasks, such as automatic target recognition and seismic signal processing. The first version of ART was "ART1", developed by Carpenter and Grossberg (1988).^{[4]}
Method of moments
One of the approaches in unsupervised learning is the method of moments. In the method of moments, the unknown parameters (of interest) in the model are related to the moments of one or more random variables, and thus, these unknown parameters can be estimated given the moments. The moments are usually estimated from samples in an empirical way. The basic moments are first and second order moments. For a random vector, the first order moment is the mean vector, and the second order moment is the covariance matrix (when the mean is zero). Higher order moments are usually represented using tensors which are the generalization of matrices to higher orders as multidimensional arrays.
In particular, the method of moments is shown to be effective in learning the parameters of latent variable models.^{[5]} Latent variable models are statistical models where in addition to the observed variables, a set of latent variables also exists which is not observed. A highly practical example of latent variable models in machine learning is the topic modeling which is a statistical model for generating the words (observed variables) in the document based on the topic (latent variable) of the document. In the topic modeling, the words in the document are generated according to different statistical parameters when the topic of the document is changed. It is shown that method of moments (tensor decomposition techniques) consistently recover the parameters of a large class of latent variable models under some assumptions.^{[5]}
The Expectation–maximization algorithm (EM) is also one of the most practical methods for learning latent variable models. However, it can get stuck in local optima, and it is not guaranteed that the algorithm will converge to the true unknown parameters of the model. Alternatively, for the method of moments, the global convergence is guaranteed under some conditions.^{[5]}
See also
 Cluster analysis
 Expectation–maximization algorithm
 Generative topographic map
 Multivariate analysis
 Radial basis function network
Notes
 ↑ Jordan, Michael I.; Bishop, Christopher M. (2004). "Neural Networks". In Allen B. Tucker. Computer Science Handbook, Second Edition (Section VII: Intelligent Systems). Boca Raton, FL: Chapman & Hall/CRC Press LLC. ISBN 158488360X.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
 ↑ Hastie,Trevor,Robert Tibshirani, Friedman,Jerome (2009). The Elements of Statistical Learning: Data mining,Inference,and Prediction. New York: Springer. pp. 485–586. ISBN 9780387848570. <templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
 ↑ Acharyya, Ranjan (2008); A New Approach for Blind Source Separation of Convolutive Sources, ISBN 9783639077971 (this book focuses on unsupervised learning with Blind Source Separation)
 ↑ Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
 ↑ ^{5.0} ^{5.1} ^{5.2} Anandkumar, Animashree; Ge, Rong; Hsu, Daniel; Kakade, Sham; Telgarsky, Matus (2014). "Tensor Decompositions for Learning Latent Variable Models" (PDF). Journal of Machine Learning Research (JMLR). 15: 2773−2832.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
Further reading
 Bousquet, O.; von Luxburg, U.; Raetsch, G., eds. (2004). Advanced Lectures on Machine Learning. SpringerVerlag. ISBN 9783540231226.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
 Duda, Richard O.; Hart, Peter E.; Stork, David G. (2001). "Unsupervised Learning and Clustering". Pattern classification (2nd ed.). Wiley. ISBN 0471056693.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
 Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
 Hinton, Geoffrey; Sejnowski, Terrence J., eds. (1999). Unsupervised Learning: Foundations of Neural Computation. MIT Press. ISBN 026258168X.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles> (This book focuses on unsupervised learning in neural networks)
 CS1 maint: Multiple names: authors list
 Articles needing cleanup from May 2010
 All articles needing cleanup
 Cleanup tagged articles without a reason field from May 2010
 Wikipedia pages needing cleanup from May 2010
 Articles lacking intext citations from February 2010
 Articles with unsourced statements from November 2011
 Machine learning