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[[File:T-SNE visualisation of word embeddings generated using 19th century literature.png|thumb|T-SNE visualisation of [[word embedding]]s generated using 19th century literature]]
[[File:T-SNE Embedding of MNIST.png|thumb|T-SNE embeddings of [[MNIST]] dataset]]
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{{Data Visualization}}
'''t-distributed stochastic neighbor embedding''' ('''t-SNE''') is a [[statistical]] method for visualizing high-dimensional data by giving each datapoint a ___location in a two or three-dimensional map. It is based on Stochastic Neighbor Embedding originally developed by
The t-SNE algorithm comprises two main stages. First, t-SNE constructs a [[probability distribution]] over pairs of high-dimensional objects in such a way that similar objects are assigned a higher probability while dissimilar points are assigned a lower probability. Second, t-SNE defines a similar probability distribution over the points in the low-dimensional map, and it minimizes the [[Kullback–Leibler divergence]] (KL divergence) between the two distributions with respect to the locations of the points in the map. While the original algorithm uses the [[Euclidean distance]] between objects as the base of its similarity metric, this can be changed as appropriate. A [[Riemannian metric|Riemannian]] variant is [[Uniform manifold approximation and projection|UMAP]].
t-SNE has been used for visualization in a wide range of applications, including [[genomics]], [[computer security]] research,<ref>{{cite journal|last=Gashi|first=I.|author2=Stankovic, V. |author3=Leita, C. |author4=Thonnard, O. |title=An Experimental Study of Diversity with Off-the-shelf AntiVirus Engines|journal=Proceedings of the IEEE International Symposium on Network Computing and Applications|year=2009|pages=4–11}}</ref> [[natural language processing]], [[music analysis]],<ref>{{cite journal|last=Hamel|first=P.|author2=Eck, D. |title=Learning Features from Music Audio with Deep Belief Networks|journal=Proceedings of the International Society for Music Information Retrieval Conference|year=2010|pages=339–344}}</ref> [[cancer research]],<ref>{{cite journal|last=Jamieson|first=A.R.|author2=Giger, M.L. |author3=Drukker, K. |author4=Lui, H. |author5=Yuan, Y. |author6=Bhooshan, N. |title=Exploring Nonlinear Feature Space Dimension Reduction and Data Representation in Breast CADx with Laplacian Eigenmaps and t-SNE|journal=Medical Physics |issue=1|year=2010|pages=339–351|doi=10.1118/1.3267037|pmid=20175497|volume=37|pmc=2807447}}</ref> [[bioinformatics]],<ref>{{cite journal|last=Wallach|first=I.|author2=Liliean, R. |title=The Protein-Small-Molecule Database, A Non-Redundant Structural Resource for the Analysis of Protein-Ligand Binding|journal=Bioinformatics |year=2009|pages=615–620|doi=10.1093/bioinformatics/btp035|volume=25|issue=5|pmid=19153135|doi-access=free}}</ref> geological ___domain interpretation,<ref>{{Cite journal|date=2019-04-01|title=A comparison of t-SNE, SOM and SPADE for identifying material type domains in geological data|url=https://www.sciencedirect.com/science/article/pii/S0098300418306010|journal=Computers & Geosciences|language=en|volume=125|pages=78–89|doi=10.1016/j.cageo.2019.01.011|issn=0098-3004|last1=Balamurali|first1=Mehala|last2=Silversides|first2=Katherine L.|last3=Melkumyan|first3=Arman|bibcode=2019CG....125...78B |s2cid=67926902|url-access=subscription}}</ref><ref>{{Cite
For a data set with ''n'' elements, t-SNE runs in {{math|O(''n''<sup>2</sup>)}} time and requires {{math|O(''n''<sup>2</sup>)}} space.<ref>{{cite
While t-SNE plots often seem to display [[cluster analysis|clusters]], the visual clusters can be influenced strongly by the chosen parameterization and therefore a good understanding of the parameters for t-SNE is necessary. Such "clusters" can be shown to even appear in non-clustered data,<ref>{{Cite web|url=https://stats.stackexchange.com/a/264647|title=K-means clustering on the output of t-SNE|website=Cross Validated|access-date=2018-04-16}}</ref> and thus may be false findings. Interactive exploration may thus be necessary to choose parameters and validate results.<ref>{{Cite journal|last1=Pezzotti|first1=Nicola|last2=Lelieveldt|first2=Boudewijn P. F.|last3=Maaten|first3=Laurens van der|last4=Hollt|first4=Thomas|last5=Eisemann|first5=Elmar|last6=Vilanova|first6=Anna|date=2017-07-01|title=Approximated and User Steerable tSNE for Progressive Visual Analytics|journal=IEEE Transactions on Visualization and Computer Graphics|language=en-US|volume=23|issue=7|pages=1739–1752|doi=10.1109/tvcg.2016.2570755|pmid=28113434|issn=1077-2626|arxiv=1512.01655|s2cid=353336}}</ref><ref>{{cite journal|url=https://distill.pub/2016/misread-tsne/|title=How to Use t-SNE Effectively|last1=Wattenberg|first1=Martin|last2=Viégas|first2=Fernanda|date=2016-10-13|journal=Distill|language=en|access-date=4 December 2017|last3=Johnson|first3=Ian|volume=1 |issue=10 |doi=10.23915/distill.00002 }}</ref> It has been demonstrated that t-SNE is often able to recover well-separated clusters, and with special parameter choices, approximates a simple form of [[spectral clustering]].<ref>{{cite arXiv|last1=Linderman|first1=George C.|last2=Steinerberger|first2=Stefan|date=2017-06-08|title=Clustering with t-SNE, provably|eprint=1706.02582|class=cs.LG}}</ref>▼
▲For a data set with ''n'' elements, t-SNE runs in {{math|O(''n''<sup>2</sup>)}} time and requires {{math|O(''n''<sup>2</sup>)}} space.<ref>{{cite web|url=https://arxiv.org/pdf/1512.01655.pdf|title=Approximated and User Steerable tSNE for Progressive Visual Analytics|last=Pezzotti|first=Nicola|access-date=31 August 2023}}</ref>
== Details ==
Given a set of <math>N</math> high-dimensional objects <math>\mathbf{x}_1, \dots, \mathbf{x}_N</math>, t-SNE first computes probabilities <math>p_{ij}</math> that are proportional to the similarity of objects <math>\mathbf{x}_i</math> and <math>\mathbf{x}_j</math>, as follows.
For <math>i \neq j</math>, define
: <math>p_{j\mid i} = \frac{\exp(-\lVert\mathbf{x}_i - \mathbf{x}_j\rVert^2 / 2\sigma_i^2)}{\sum_{k \neq i} \exp(-\lVert\mathbf{x}_i - \mathbf{x}_k\rVert^2 / 2\sigma_i^2)}</math>
and set <math>p_{i\mid i} = 0</math>.
Note
As
Now define
: <math>p_{ij} = \frac{p_{j\mid i} + p_{i\mid j}}{2N}</math>
This is motivated because <math>p_{i}</math> and <math>p_{j}</math> from the N samples are estimated as 1/N, so the conditional probability can be written as <math>p_{i\mid j} = Np_{ij}</math> and <math>p_{j\mid i} = Np_{ji}</math> . Since <math> p_{ij} = p_{ji}</math>, you can obtain previous formula.
Also note that <math>p_{ii} = 0 </math> and <math>\sum_{i, j} p_{ij} = 1</math>.
The bandwidth of the [[Gaussian kernel]]s <math>\sigma_i</math> is set in such a way that the [[Entropy (information theory)|entropy]] of the conditional distribution equals a predefined entropy using the [[bisection method]]. As a result, the bandwidth is adapted to the [[density]] of the data: smaller values of <math>\sigma_i</math> are used in denser parts of the data space. The entropy increases with the [[perplexity]] of this distribution <math>P_i</math>; this relation is seen as
: <math>Perp(P_i) = 2^{H(P_i)}</math>
Since the Gaussian kernel uses the Euclidean distance <math>\lVert x_i-x_j \rVert</math>, it is affected by the [[curse of dimensionality]], and in high dimensional data when distances lose the ability to discriminate, the <math>p_{ij}</math> become too similar (asymptotically, they would converge to a constant). It has been proposed to adjust the distances with a power transform, based on the [[intrinsic dimension]] of each point, to alleviate this.<ref>{{Cite conference|last1=Schubert|first1=Erich|last2=Gertz|first2=Michael|date=2017-10-04|title=Intrinsic t-Stochastic Neighbor Embedding for Visualization and Outlier Detection|conference=SISAP 2017 – 10th International Conference on Similarity Search and Applications|pages=188–203|doi=10.1007/978-3-319-68474-1_13}}</ref>▼
where <math>H(P_i)</math> is the Shannon entropy <math>H(P_i)=-\sum_jp_{j|i}\log_2p_{j|i}.</math>
The perplexity is a hand-chosen parameter of t-SNE, and as the authors state, "perplexity can be interpreted as a smooth measure of the effective number of neighbors. The performance of SNE is fairly robust to changes in the perplexity, and typical values are between 5 and 50.".<ref name=MaatenHinton/>
▲Since the Gaussian kernel uses the [[Euclidean distance]] <math>\lVert x_i-x_j \rVert</math>, it is affected by the [[curse of dimensionality]], and in high dimensional data when distances lose the ability to discriminate, the <math>p_{ij}</math> become too similar (asymptotically, they would converge to a constant). It has been proposed to adjust the distances with a power transform, based on the [[intrinsic dimension]] of each point, to alleviate this.<ref>{{Cite conference|last1=Schubert|first1=Erich|last2=Gertz|first2=Michael|date=2017-10-04|title=Intrinsic t-Stochastic Neighbor Embedding for Visualization and Outlier Detection|conference=SISAP 2017 – 10th International Conference on Similarity Search and Applications|pages=188–203|doi=10.1007/978-3-319-68474-1_13}}</ref>
t-SNE aims to learn a <math>d</math>-dimensional map <math>\mathbf{y}_1, \dots, \mathbf{y}_N</math> (with <math>\mathbf{y}_i \in \mathbb{R}^d</math> and <math>d</math> typically chosen as 2 or 3) that reflects the similarities <math>p_{ij}</math> as well as possible. To this end, it measures similarities <math>q_{ij}</math> between two points in the map <math>\mathbf{y}_i</math> and <math>\mathbf{y}_j</math>, using a very similar approach.
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and set <math> q_{ii} = 0 </math>.
Herein a heavy-tailed [[Student t-distribution]] (with one-degree of freedom, which is the same as a [[Cauchy distribution]]) is used to measure similarities between low-dimensional points in order to allow dissimilar objects to be modeled far apart in the map.
The locations of the points <math>\mathbf{y}_i</math> in the map are determined by minimizing the (non-symmetric) [[Kullback–Leibler divergence]] of the distribution <math>P</math> from the distribution <math>Q</math>, that is:
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The minimization of the Kullback–Leibler divergence with respect to the points <math>\mathbf{y}_i</math> is performed using [[gradient descent]].
The result of this optimization is a map that reflects the similarities between the high-dimensional inputs.
== Output ==
▲While t-SNE plots often seem to display [[cluster analysis|clusters]], the visual clusters can be
== Software ==
* A C++ implementation of Barnes-Hut is available on the [https://github.com/lvdmaaten/bhtsne github account] of one of the original authors.
* The R package [https://CRAN.R-project.org/package=Rtsne Rtsne] implements t-SNE in [[R (programming language)|R]].
* [[ELKI]] contains tSNE, also with Barnes-Hut approximation
* [[scikit-learn]], a popular machine learning library in Python implements t-SNE with both exact solutions and the Barnes-Hut approximation.
* Tensorboard, the visualization kit associated with [[TensorFlow]], also implements t-SNE ([https://projector.tensorflow.org/ online version])
* The [[Julia (programming language)|Julia]] package [https://juliapackages.com/p/tsne TSne] implements t-SNE
== References ==
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== External links ==
* {{Cite journal |last1=Wattenberg |first1=Martin |last2=Viégas |first2=Fernanda |last3=Johnson |first3=Ian |date=2016-10-13 |title=How to Use t-SNE Effectively |url=https://distill.pub/2016/misread-tsne/ |journal=Distill |language=en |volume=1 |issue=10 |pages=e2 |doi=10.23915/distill.00002 |issn=2476-0757|doi-access=free }}. Interactive demonstration and tutorial.
* [https://www.youtube.com/watch?v=RJVL80Gg3lA Visualizing Data Using t-SNE], Google Tech Talk about t-SNE
* [https://lvdmaaten.github.io/tsne/ Implementations of t-SNE in various languages], A link collection maintained by Laurens van der Maaten
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