Unsupervised learning
Part of a series on 
Machine learning and data mining 

Paradigms

Problems

Learning with humans

Model diagnostics


Unsupervised learning is a type of algorithm that learns patterns from untagged data. The goal is that through mimicry, which is an important mode of learning in people, the machine is forced to build a concise representation of its world and then generate imaginative content from it.
In contrast to supervised learning where data is tagged by an expert, e.g. tagged as a "ball" or "fish", unsupervised methods exhibit selforganization that captures patterns as probability densities ^{[1]} or a combination of neural feature preferences encoded in the machine's weights and activations. The other levels in the supervision spectrum are reinforcement learning where the machine is given only a numerical performance score as guidance, and semisupervised learning where a small portion of the data is tagged.
Neural networks
Tasks vs. methods
Neural network tasks are often categorized as discriminative (recognition) or generative (imagination). Often but not always, discriminative tasks use supervised methods and generative tasks use unsupervised (see Venn diagram); however, the separation is very hazy. For example, object recognition favors supervised learning but unsupervised learning can also cluster objects into groups. Furthermore, as progress marches onward some tasks employ both methods, and some tasks swing from one to another. For example, image recognition started off as heavily supervised, but became hybrid by employing unsupervised pretraining, and then moved towards supervision again with the advent of dropout, relu, and adaptive learning rates.
Training
During the learning phase, an unsupervised network tries to mimic the data it's given and uses the error in its mimicked output to correct itself (i.e. correct its weights & biases). Sometimes the error is expressed as a low probability that the erroneous output occurs, or it might be expressed as an unstable high energy state in the network.
In contrast to supervised methods' dominant use of backpropagation, unsupervised learning also employs other methods including: Hopfield learning rule, Boltzmann learning rule, Contrastive Divergence, Wake Sleep, Variational Inference, Maximum Likelihood, Maximum A Posteriori, Gibbs Sampling, and backpropagating reconstruction errors or hidden state reparameterizations. See the table below for more details.
Energy
An energy function is a macroscopic measure of a network's activation state. In Boltzmann machines, it plays the role of the Cost function. This analogy with physics is inspired by Ludwig Boltzmann's analysis of a gas' macroscopic energy from the microscopic probabilities of particle motion $p\propto e^{E/kT}$, where k is the Boltzmann constant and T is temperature. In the RBM network the relation is $p=e^{E}/Z$,^{[2]} where $p$ and $E$ vary over every possible activation pattern and $\textstyle {Z=\sum _{\scriptscriptstyle {\text{All Patterns}}}e^{E({\text{pattern}})}}$. To be more precise, $p(a)=e^{E(a)}/Z$, where $a$ is an activation pattern of all neurons (visible and hidden). Hence, early neural networks bear the name Boltzmann Machine. Paul Smolensky calls $E\,$ the Harmony. A network seeks low energy which is high Harmony.
Networks
This table shows connection diagrams of various unsupervised networks, the details of which will be given in the section Comparison of Network. Circles are neurons and edges between them are connection weights. As network design changes, features are added on to enable new capabilities or removed to make learning faster. For instance, neurons change between deterministic (Hopfield) and stochastic (Boltzmann) to allow robust output, weights are removed within a layer (RBM) to hasten learning, or connections are allowed to become asymmetric (Helmholtz).
Hopfield  Boltzmann  RBM  Stacked Boltzmann  Helmholtz  Autoencoder  VAE 

A network based on magnetic domains in iron with a single selfconnected layer. It can be used as a content addressable memory.  Network is separated into 2 layers (hidden vs. visible), but still using symmetric 2way weights. Following Boltzmann's thermodynamics, individual probabilities give rise to macroscopic energies.  Restricted Boltzmann Machine. This is a Boltzmann machine where lateral connections within a layer are prohibited to make analysis tractable.  This network has multiple RBM's to encode a hierarchy of hidden features. After a single RBM is trained, another blue hidden layer (see left RBM) is added, and the top 2 layers are trained as a red & blue RBM. Thus the middle layers of an RBM acts as hidden or visible, depending on the training phase it's in.  Instead of the bidirectional symmetric connection of the stacked Boltzmann machines, we have separate oneway connections to form a loop. It does both generation and discrimination.  A feed forward network that aims to find a good middle layer representation of its input world. This network is deterministic, so it's not as robust as its successor the VAE.  Applies Variational Inference to the Autoencoder. The middle layer is a set of means & variances for Gaussian distributions. The stochastic nature allows for more robust imagination than the deterministic autoencoder. 
Of the networks bearing people's names, only Hopfield worked directly with neural networks. Boltzmann and Helmholtz came before artificial neural networks, but their work in physics and physiology inspired the analytical methods that were used.
History
1969  Perceptrons by Minsky & Papert shows a perceptron without hidden layers fails on XOR 
1970s  (approximate dates) AI winter I 
1974  Ising magnetic model proposed by WA Little for cognition 
1980  Fukushima introduces the neocognitron, which is later called a convolution neural network. It is mostly used in SL, but deserves a mention here. 
1982  Ising variant Hopfield net described as CAMs and classifiers by John Hopfield. 
1983  Ising variant Boltzmann machine with probabilistic neurons described by Hinton & Sejnowski following Sherington & Kirkpatrick's 1975 work. 
1986  Paul Smolensky publishes Harmony Theory, which is an RBM with practically the same Boltzmann energy function. Smolensky did not give an practical training scheme. Hinton did in mid2000s 
1995  Schmidthuber introduces the LSTM neuron for languages. 
1995  Dayan & Hinton introduces Helmholtz machine 
19952005  (approximate dates) AI winter II 
2013  Kingma, Rezende, & co. introduced Variational Autoencoders as Bayesian graphical probability network, with neural nets as components. 
Specific Networks
Here, we highlight some characteristics of select networks. The details of each are given in the comparison table below.
 Hopfield Network
 Ferromagnetism inspired Hopfield networks. A neuron correspond to an iron domain with binary magnetic moments Up and Down, and neural connections correspond to the domain's influence on each other. Symmetric connections enable a global energy formulation. During inference the network updates each state using the standard activation step function. Symmetric weights and the right energy functions guarantees convergence to a stable activation pattern. Asymmetric weights are difficult to analyze. Hopfield nets are used as Content Addressable Memories (CAM).
 Boltzmann Machine
 These are stochastic Hopfield nets. Their state value is sampled from this pdf as follows: suppose a binary neuron fires with the Bernoulli probability p(1) = 1/3 and rests with p(0) = 2/3. One samples from it by taking a UNIFORMLY distributed random number y, and plugging it into the inverted cumulative distribution function, which in this case is the step function thresholded at 2/3. The inverse function = { 0 if x <= 2/3, 1 if x > 2/3 }
 Sigmoid Belief Net
 Introduced by Radford Neal in 1992, this network applies ideas from probabilistic graphical models to neural networks. A key difference is that nodes in graphical models have preassigned meanings, whereas Belief Net neurons' features are determined after training. The network is a sparsely connected directed acyclic graph composed of binary stochastic neurons. The learning rule comes from Maximum Likelihood on p(X): Δw_{ij} $\propto$ s_{j} * (s_{i}  p_{i}), where p_{i} = 1 / ( 1 + e^{weighted inputs into neuron i} ). s_{j}'s are activations from an unbiased sample of the posterior distribution and this is problematic due to the Explaining Away problem raised by Judea Perl. Variational Bayesian methods uses a surrogate posterior and blatantly disregard this complexity.
 Deep Belief Network
 Introduced by Hinton, this network is a hybrid of RBM and Sigmoid Belief Network. The top 2 layers is an RBM and the second layer downwards form a sigmoid belief network. One trains it by the stacked RBM method and then throw away the recognition weights below the top RBM. As of 2009, 34 layers seems to be the optimal depth.^{[3]}
 Helmholtz machine
 These are early inspirations for the Variational Auto Encoders. It's 2 networks combined into one—forward weights operates recognition and backward weights implements imagination. It is perhaps the first network to do both. Helmholtz did not work in machine learning but he inspired the view of "statistical inference engine whose function is to infer probable causes of sensory input" (3). the stochastic binary neuron outputs a probability that its state is 0 or 1. The data input is normally not considered a layer, but in the Helmholtz machine generation mode, the data layer receives input from the middle layer has separate weights for this purpose, so it is considered a layer. Hence this network has 3 layers.
 Variational autoencoder
 These are inspired by Helmholtz machines and combines probability network with neural networks. An Autoencoder is a 3layer CAM network, where the middle layer is supposed to be some internal representation of input patterns. The encoder neural network is a probability distribution q_{φ}(z given x) and the decoder network is p_{θ}(x given z). The weights are named phi & theta rather than W and V as in Helmholtz—a cosmetic difference. These 2 networks here can be fully connected, or use another NN scheme.
Comparison of networks
Hopfield  Boltzmann  RBM  Stacked RBM  Helmholtz  Autoencoder  VAE  

Usage & notables  CAM, traveling salesman problem  CAM. The freedom of connections makes this network difficult to analyze.  pattern recognition. used in MNIST digits and speech.  recognition & imagination. trained with unsupervised pretraining and/or supervised fine tuning.  imagination, mimicry  language: creative writing, translation. vision: enhancing blurry images  generate realistic data 
Neuron  deterministic binary state. Activation = { 0 (or 1) if x is negative, 1 otherwise }  stochastic binary Hopfield neuron  ← same. (extended to realvalued in mid 2000s)  ← same  ← same  language: LSTM. vision: local receptive fields. usually real valued relu activation.  middle layer neurons encode means & variances for Gaussians. In run mode (inference), the output of the middle layer are sampled values from the Gaussians. 
Connections  1layer with symmetric weights. No selfconnections.  2layers. 1hidden & 1visible. symmetric weights.  ← same. no lateral connections within a layer.  top layer is undirected, symmetric. other layers are 2way, asymmetric.  3layers: asymmetric weights. 2 networks combined into 1.  3layers. The input is considered a layer even though it has no inbound weights. recurrent layers for NLP. feedforward convolutions for vision. input & output have the same neuron counts.  3layers: input, encoder, distribution sampler decoder. the sampler is not considered a layer (e) 
Inference & energy  Energy is given by Gibbs probability measure :$E={\frac {1}{2}}\sum _{i,j}{w_{ij}{s_{i}}{s_{j}}}+\sum _{i}{\theta _{i}}{s_{i}}$  ← same  ← same  minimize KL divergence  inference is only feedforward. previous UL networks ran forwards AND backwards  minimize error = reconstruction error  KLD  
Training  Δw_{ij} = s_{i}*s_{j}, for +1/1 neuron  Δw_{ij} = e*(p_{ij}  p'_{ij}). This is derived from minimizing KLD. e = learning rate, p' = predicted and p = actual distribution.  Δw_{ij} = e*( < v_{i} h_{j} >_{data}  < v_{i} h_{j} >_{equilibrium} ). This is a form of contrastive divergence w/ Gibbs Sampling. "<>" are expectations.  ← similar. train 1layer at a time. approximate equilibrium state with a 3segment pass. no back propagation.  wakesleep 2 phase training  back propagate the reconstruction error  reparameterize hidden state for backprop 
Strength  resembles physical systems so it inherits their equations  ← same. hidden neurons act as internal representatation of the external world  faster more practical training scheme than Boltzmann machines  trains quickly. gives hierarchical layer of features  mildly anatomical. analyzable w/ information theory & statistical mechanics  
Weakness  hard to train due to lateral connections  equilibrium requires too many iterations  integer & realvalued neurons are more complicated. 
Hebbian Learning, ART, SOM
The classical example of unsupervised learning in the study of neural networks is Donald Hebb's principle, that is, neurons that fire together wire together.^{[4]} In Hebbian learning, the connection is reinforced irrespective of an error, but is exclusively a function of the coincidence between action potentials between the two neurons.^{[5]} A similar version that modifies synaptic weights takes into account the time between the action potentials (spiketimingdependent plasticity or STDP). Hebbian Learning has been hypothesized to underlie a range of cognitive functions, such as pattern recognition and experiential learning.
Among neural network models, the selforganizing map (SOM) and adaptive resonance theory (ART) are commonly used in 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 used for many pattern recognition tasks, such as automatic target recognition and seismic signal processing.^{[6]}
Probabilistic methods
Two of the main methods used in unsupervised learning are principal component and cluster analysis. Cluster analysis is used in unsupervised learning to group, or segment, datasets with shared attributes in order to extrapolate algorithmic relationships.^{[7]} Cluster analysis is a branch of machine learning that groups the data that has not been labelled, classified or categorized. Instead of responding to feedback, cluster analysis identifies commonalities in the data and reacts based on the presence or absence of such commonalities in each new piece of data. This approach helps detect anomalous data points that do not fit into either group.
A central application of unsupervised learning is in the field of density estimation in statistics,^{[8]} though unsupervised learning encompasses many other domains involving summarizing and explaining data features. It can be contrasted with supervised learning by saying that whereas supervised learning intends to infer a conditional probability distribution conditioned on the label of input data; unsupervised learning intends to infer an a priori probability distribution .
Approaches
Some of the most common algorithms used in unsupervised learning include: (1) Clustering, (2) Anomaly detection, (3) Approaches for learning latent variable models. Each approach uses several methods as follows:
 Clustering methods include: hierarchical clustering,^{[9]} kmeans,^{[10]} mixture models, DBSCAN, and OPTICS algorithm
 Anomaly detection methods include: Local Outlier Factor, and Isolation Forest
 Approaches for learning latent variable models such as Expectation–maximization algorithm (EM), Method of moments, and Blind signal separation techniques (Principal component analysis, Independent component analysis, Nonnegative matrix factorization, Singular value decomposition)
Method of moments
One of the statistical approaches for 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 empirically. 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. 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.^{[11]}
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. In contrast, for the method of moments, the global convergence is guaranteed under some conditions.
See also
 Automated machine learning
 Cluster analysis
 Anomaly detection
 Expectation–maximization algorithm
 Generative topographic map
 Metalearning (computer science)
 Multivariate analysis
 Radial basis function network
 Weak supervision
References
 ^ Hinton & Sejnowski 1999
 ^ Hinton, G. (2012). "A Practical Guide to Training Restricted Boltzmann Machines" (PDF). Neural Networks: Tricks of the Trade. Lecture Notes in Computer Science. Vol. 7700. Springer. pp. 599–619. doi:10.1007/9783642352898_32. ISBN 9783642352898.
 ^ Hinton, Geoffrey (September 2009). "Deep Belief Nets" (video).
 ^ Buhmann, J.; Kuhnel, H. (1992). "Unsupervised and supervised data clustering with competitive neural networks". [Proceedings 1992] IJCNN International Joint Conference on Neural Networks. Vol. 4. IEEE. pp. 796–801. doi:10.1109/ijcnn.1992.227220. ISBN 0780305590. S2CID 62651220.
 ^ ComesañaCampos, Alberto; BouzaRodríguez, José Benito (June 2016). "An application of Hebbian learning in the design process decisionmaking". Journal of Intelligent Manufacturing. 27 (3): 487–506. doi:10.1007/s108450140881z. ISSN 09565515. S2CID 207171436.
 ^ Carpenter, G.A. & Grossberg, S. (1988). "The ART of adaptive pattern recognition by a selforganizing neural network" (PDF). Computer. 21 (3): 77–88. doi:10.1109/2.33. S2CID 14625094.
 ^ Roman, Victor (20190421). "Unsupervised Machine Learning: Clustering Analysis". Medium. Retrieved 20191001.
 ^ Jordan, Michael I.; Bishop, Christopher M. (2004). "7. Intelligent Systems §Neural Networks". In Tucker, Allen B. (ed.). Computer Science Handbook (2nd ed.). Chapman & Hall/CRC Press. doi:10.1201/9780203494455. ISBN 158488360X.
 ^ Hastie, Tibshirani & Friedman 2009, pp. 485–586
 ^ Garbade, Dr Michael J. (20180912). "Understanding Kmeans Clustering in Machine Learning". Medium. Retrieved 20191031.
 ^ Anandkumar, Animashree; Ge, Rong; Hsu, Daniel; Kakade, Sham; Telgarsky, Matus (2014). "Tensor Decompositions for Learning Latent Variable Models" (PDF). Journal of Machine Learning Research. 15: 2773–2832. arXiv:1210.7559. Bibcode:2012arXiv1210.7559A.
Further reading
 Bousquet, O.; von Luxburg, U.; Raetsch, G., eds. (2004). Advanced Lectures on Machine Learning. Springer. ISBN 9783540231226.
 Duda, Richard O.; Hart, Peter E.; Stork, David G. (2001). "Unsupervised Learning and Clustering". Pattern classification (2nd ed.). Wiley. ISBN 0471056693.
 Hastie, Trevor; Tibshirani, Robert; Friedman, Jerome (2009). "Unsupervised Learning". The Elements of Statistical Learning: Data mining, Inference, and Prediction. Springer. pp. 485–586. doi:10.1007/9780387848587_14. ISBN 9780387848570.
 Hinton, Geoffrey; Sejnowski, Terrence J., eds. (1999). Unsupervised Learning: Foundations of Neural Computation. MIT Press. ISBN 026258168X.