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| [[File:RecentVotes.svg|thumb|400px|An example of classical multidimensional scaling applied to voting patterns in the [[United States House of Representatives]]]] | | [http://www.google.co.uk/search?hl=en&gl=us&tbm=nws&q=Adrianne&gs_l=news Adrianne] is what you is able to call me but We all don't like when everyone use my full user name. What I love doing is fish and I'll be creating something else along with it. Filing is certainly my day job proper but soon I'll getting on my own. Vermont needs always been my does not place and I are blessed with everything that I have to have here. Go to the group website to find apart more: http://circuspartypanama.com<br><br>Here is my weblog - [http://circuspartypanama.com clash of clans hack tool download no survey] |
| '''Multidimensional scaling''' ('''MDS''') is a means of visualizing the level of similarity of individual cases of a dataset. It refers to a set of related [[Ordination (statistics)|ordination]] techniques used in [[information visualization]], in particular to display the information contained in a [[distance matrix]]. An MDS [[algorithm]] aims to place each object in ''N''-[[dimension]]al space such that the between-object distances are preserved as well as possible. Each object is then assigned [[coordinate]]s in each of the ''N'' dimensions. The number of dimensions of an MDS plot ''N'' can exceed 2 and is specified [[a priori and a posteriori|a priori]]. Choosing ''N''=2 optimizes the object locations for a two-dimensional [[scatterplot]].<ref name="borg">{{cite book |author=Borg, I., Groenen, P. |title=Modern Multidimensional Scaling: theory and applications |publisher=Springer-Verlag |location=New York |year=2005 |pages=207–212 |edition=2nd |isbn=0-387-94845-7 }}</ref>
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| ==Types==
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| MDS algorithms fall into a [[Taxonomy (general)|taxonomy]], depending on the meaning of the input matrix:
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| ; Classical multidimensional scaling: Also known as Principal Coordinates Analysis, Torgerson Scaling or Torgerson–Gower scaling. Takes an input matrix giving dissimilarities between pairs of items and outputs a coordinate matrix whose configuration minimizes a loss function called ''strain''.<ref name="borg"/>
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| ; Metric multidimensional scaling: A superset of classical MDS that generalizes the optimization procedure to a variety of loss functions and input matrices of known distances with weights and so on. A useful loss function in this context is called ''stress'', which is often minimized using a procedure called [[stress majorization]].
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| ; Non-metric multidimensional scaling: In contrast to metric MDS, non-metric MDS finds both a [[non-parametric]] [[monotonic]] relationship between the dissimilarities in the item-item matrix and the Euclidean distances between items, and the location of each item in the low-dimensional space. The relationship is typically found using [[isotonic regression]].
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| ;*[[Louis Guttman]]'s '''smallest space analysis''' (SSA) is an example of a non-metric MDS procedure.
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| ; Generalized multidimensional scaling: An extension of metric multidimensional scaling, in which the target space is an arbitrary smooth non-Euclidean space. In cases where the dissimilarities are distances on a surface and the target space is another surface, GMDS allows finding the minimum-distortion embedding of one surface into another.<ref name="bron">{{cite journal |author=Bronstein AM, Bronstein MM, Kimmel R |title=Generalized multidimensional scaling: a framework for isometry-invariant partial surface matching |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=103 |issue=5 |pages=1168–72 |date=January 2006 |pmid=16432211 |pmc=1360551 |doi=10.1073/pnas.0508601103 |url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=16432211}}</ref>
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| ==Details==
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| The data to be analyzed is a collection of <math>I</math> objects (colors, faces, stocks, . . .) on which a ''distance function'' is defined,
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| :δ<sub>''i,j''</sub> := distance between ''i'' th and ''j'' th objects.
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| These distances are the entries of the ''dissimilarity matrix''
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| :<math>\Delta :=
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| \begin{pmatrix}
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| \delta_{1,1} & \delta_{1,2} & \cdots & \delta_{1,I} \\
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| \delta_{2,1} & \delta_{2,2} & \cdots & \delta_{2,I} \\
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| \vdots & \vdots & & \vdots \\
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| \delta_{I,1} & \delta_{I,2} & \cdots & \delta_{I,I}
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| \end{pmatrix}.
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| </math>
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| The goal of MDS is, given Δ, to find <math>I</math> vectors
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| <math>x_1,\ldots,x_I \in \mathbb{R}^N</math> such that
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| :<math>\|x_i - x_j\| \approx \delta_{i,j}</math> for all <math>i,j\in I</math>,
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| where <math>\|\cdot\|</math> is a [[Norm (mathematics)|vector norm]]. In classical MDS, this norm is the [[Euclidean distance]], but, in a broader sense, it may be a [[metric (mathematics)|metric]] or arbitrary distance function.<ref name="Kruskal">[[Joseph Kruskal|Kruskal, J. B.]], and Wish, M. (1978), ''Multidimensional Scaling'', Sage University Paper series on Quantitative Application in the Social Sciences, 07-011. Beverly Hills and London: Sage Publications.</ref>
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| In other words, MDS attempts to find an [[embedding]] from the <math>I</math> objects into '''R'''<sup>''N''</sup> such that distances are preserved. If the dimension ''N'' is chosen to be 2 or 3, we may plot the vectors ''x<sub>i</sub>'' to obtain a visualization of the similarities between the <math>I</math> objects. Note that the vectors ''x<sub>i</sub>'' are not unique: With the Euclidean distance, they may be arbitrarily translated, rotated, and reflected, since these transformations do not change the pairwise distances <math>\|x_i - x_j\|</math>.
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| (Note: The symbol <math>\mathbb{R}</math> indicates the set of [[real numbers]], and the notation <math>\mathbb{R}^N</math> refers to the Cartesian product of n copies of R, which is an n-dimensional vector space over the field of the real numbers.)
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| There are various approaches to determining the vectors ''x<sub>i</sub>''. Usually, MDS is formulated as an [[optimization (mathematics)|optimization problem]], where <math>(x_1,\ldots,x_I)</math> is found as a minimizer of some cost function, for example,
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| :<math> \min_{x_1,\ldots,x_I} \sum_{i<j} ( \|x_i - x_j\| - \delta_{i,j} )^2. \, </math>
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| A solution may then be found by numerical optimization techniques. For some particularly chosen cost functions, minimizers can be stated analytically in terms of matrix [[Eigendecomposition of a matrix|eigendecompositions]].{{citation needed|date=September 2012}}
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| ==Procedure==
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| {{Confusing|date=November 2009}}
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| There are several steps in conducting MDS research:
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| # '''Formulating the problem''' – What variables do you want to compare? How many variables do you want to compare? More than 20 is often considered cumbersome. {{Citation needed|date=May 2012}} Fewer than 8 (4 pairs) will not give valid results. {{Citation needed|date=May 2012}} What purpose is the study to be used for?
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| # '''Obtaining input data''' – Respondents are asked a series of questions. For each product pair, they are asked to rate similarity (usually on a 7 point [[Likert scale]] from very similar to very dissimilar). The first question could be for Coke/Pepsi for example, the next for Coke/Hires rootbeer, the next for Pepsi/Dr Pepper, the next for Dr Pepper/Hires rootbeer, etc. The number of questions is a function of the number of brands and can be calculated as <math>Q = N (N - 1) / 2</math> where ''Q'' is the number of questions and ''N'' is the number of brands. This approach is referred to as the “Perception data : direct approach”. There are two other approaches. There is the “Perception data : derived approach” in which products are decomposed into attributes that are rated on a [[semantic differential]] scale. The other is the “Preference data approach” in which respondents are asked their preference rather than similarity.
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| # '''Running the MDS statistical program''' – Software for running the procedure is available in many software for statistics. Often there is a choice between Metric MDS (which deals with interval or ratio level data), and Nonmetric MDS (which deals with ordinal data).
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| # '''Decide number of dimensions''' – The researcher must decide on the number of dimensions they want the computer to create. The more dimensions, the better the statistical fit, but the more difficult it is to interpret the results.
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| # '''Mapping the results and defining the dimensions''' – The statistical program (or a related module) will map the results. The map will plot each product (usually in two-dimensional space). The proximity of products to each other indicate either how similar they are or how preferred they are, depending on which approach was used. How the dimensions of the embedding actually correspond to dimensions of system behavior, however, are not necessarily obvious. Here, a subjective judgment about the correspondence can be made (see [[perceptual mapping]]).
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| # '''Test the results for reliability and validity''' – Compute [[R-squared]] to determine what proportion of variance of the scaled data can be accounted for by the MDS procedure. An R-square of 0.6 is considered the minimum acceptable level. {{Citation needed|date=February 2011}} An R-square of 0.8 is considered good for metric scaling and .9 is considered good for non-metric scaling. Other possible tests are Kruskal’s Stress, split data tests, data stability tests (i.e., eliminating one brand), and test-retest reliability.
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| # '''Report the results comprehensively''' – Along with the mapping, at least distance measure (e.g., [[Sorenson index]], [[Jaccard index]]) and reliability (e.g., stress value) should be given. It is also very advisable to give the algorithm (e.g., Kruskal, Mather), which is often defined by the program used (sometimes replacing the algorithm report), if you have given a start configuration or had a random choice, the number of runs, the assessment of dimensionality, the [[Monte Carlo|Monte Carlo method]] results, the number of iterations, the assessment of stability, and the proportional variance of each axis (r-square).
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| ==Applications==
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| Applications include [[scientific visualisation]] and [[data mining]] in fields such as [[cognitive science]], [[information science]], [[psychophysics]], [[psychometrics]], [[marketing]] and [[ecology]].
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| New applications arise in the scope of autonomous wireless nodes that populate a space or an area. MDS may apply as a real time enhanced approach to monitoring and managing such populations.
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| Furthermore, MDS has been used extensively in [[geostatistics]], for modeling the spatial variability of the patterns of an image (by representing them as points in a lower-dimensional space),<ref>Honarkhah, M and Caers, J, 2010, ''[http://dx.doi.org/10.1007/s11004-010-9276-7 Stochastic Simulation of Patterns Using Distance-Based Pattern Modeling]'', Mathematical Geosciences, 42: 487–517</ref> and [[natural language processing]], for modeling the semantic and affective relatedness of natural language concepts (by representing them as points in a 100-dimensional vector space).<ref>Cambria, E, Song, Y, Wang, H and Howard, N, 2013, '[http://dx.doi.org/10.1109/MIS.2012.118 Semantic multi-dimensional scaling for open-domain sentiment analysis]", IEEE Intelligent Systems</ref>
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| ===Marketing===
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| In [[marketing]], MDS is a statistical technique for taking the preferences and perceptions of respondents and representing them on a visual grid, called [[perceptual mapping|perceptual maps]].
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| ===Comparison and advantages===
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| Potential customers are asked to compare pairs of [[product (business)|product]]s and make judgments about their similarity. Whereas other techniques (such as [[factor analysis]], [[discriminant analysis (in marketing)|discriminant analysis]], and [[conjoint analysis (in marketing)|conjoint analysis]]) obtain underlying dimensions from responses to product attributes identified by the researcher, MDS obtains the underlying dimensions from respondents’ judgments about the similarity of products. This is an important advantage. {{Citation needed|date=May 2012}} It does not depend on researchers’ judgments. It does not require a list of attributes to be shown to the respondents. The underlying dimensions come from respondents’ judgments about pairs of products. Because of these advantages, MDS is the most common technique used in perceptual mapping. {{Citation needed|date=May 2012}}
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| ==Implementations==
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| * ''cmdscale'' in ''[[R (programming language)|R]]
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| * ''NMS'' in [http://www.pcord.com PC-ORD, Multivariate Analysis of Ecological Data]
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| * [[Orange (software)|Orange]], a free data mining software suite, module [http://www.ailab.si/orange/doc/modules/orngMDS.htm orngMDS]
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| * [http://www.uv.es/visualstats/Book ViSta] has implementations of MDS by Forrest W. Young. Interactive graphics allow exploring the results of MDS in detail.
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| * [http://www.usabilitest.com/CardSorting usabiliTEST's Online Card Sorting] software is utilizing MDS to plot the data collected from the participants of usability tests.
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| == See also ==
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| {{Commons category|Multidimensional scaling}}
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| * [[Positioning (marketing)]]
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| * [[Perceptual mapping]]
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| * [[Product management]]
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| * [[Marketing]]
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| * [[Generalized multidimensional scaling]] (GMDS)
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| * [[Data clustering]]
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| * [[Factor analysis]]
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| * [[Discriminant analysis]]
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| * [[Nonlinear dimensionality reduction]]
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| * [[Distance geometry]]
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| * [[Cayley–Menger determinant]]
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| * [[Dimension reduction]]
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| == Bibliography ==
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| {{reflist}}
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| {{refbegin}}
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| *{{cite book |author=Cox, T.F., Cox, M.A.A. |title=Multidimensional Scaling |publisher=Chapman and Hall |year=2001 }}
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| *{{cite book |author=Coxon, Anthony P.M. |title=The User's Guide to Multidimensional Scaling. With special reference to the MDS(X) library of Computer Programs |publisher=Heinemann Educational Books |location=London |year=1982 }}
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| *{{cite journal |author=Green, P. |title=Marketing applications of MDS: Assessment and outlook |journal=Journal of Marketing |volume=39 |pages=24–31 |date=January 1975 |doi=10.2307/1250799 |issue=1 }}
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| *{{cite book |author=McCune, B. and Grace, J.B. |title=Analysis of Ecological Communities |publisher=MjM Software Design |location=Oregon, Gleneden Beach |year=2002 |isbn=0-9721290-0-6 }}
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| *{{cite book |author=Torgerson, Warren S. |title=Theory & Methods of Scaling |publisher=Wiley |location=New York |year=1958 |isbn=0-89874-722-8 }}
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| {{refend}}
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| == External links ==
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| * [http://www.mathpsyc.uni-bonn.de/doc/delbeke/delbeke.htm An elementary introduction to multidimensional scaling]
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| * [http://www.newmdsx.com/ NewMDSX: Multidimensional Scaling Software]
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| * [http://www.granular.com/MDS/ MDS page]
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| * [http://codingplayground.blogspot.com/2009/05/multidimension-scaling.html MDS in C++] by Antonio Gulli
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| * [http://orange.biolab.si/doc/modules/orngMDS.htm The orngMDS module] for MDS from [[Orange (software)]]
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| {{DEFAULTSORT:Multidimensional Scaling}}
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| [[Category:Multivariate statistics]]
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| [[Category:Market research]]
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| [[Category:Psychometrics]]
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