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Nonclassical multidimensional scaling

`Y = mdscale(D,p)`

[Y,stress] = mdscale(D,p)

[Y,stress,disparities] = mdscale(D,p)

[...] = mdscale(D,p,'* Name*',

`value`

`Y = mdscale(D,p)`

performs nonmetric multidimensional
scaling on the *n*-by-*n* dissimilarity
matrix `D`

, and returns `Y`

, a configuration
of *n* points (rows) in `p`

dimensions
(columns). The Euclidean distances between points in `Y`

approximate
a monotonic transformation of the corresponding dissimilarities in `D`

.
By default, `mdscale`

uses Kruskal's normalized stress1
criterion.

You can specify `D`

as either a full *n*-by-*n* matrix,
or in upper triangle form such as is output by `pdist`

.
A full dissimilarity matrix must be real and symmetric, and have zeros
along the diagonal and non-negative elements everywhere else. A dissimilarity
matrix in upper triangle form must have real, non-negative entries. `mdscale`

treats `NaN`

s
in `D`

as missing values, and ignores those elements. `Inf`

is
not accepted.

You can also specify `D`

as a full similarity
matrix, with ones along the diagonal and all other elements less than
one. `mdscale`

transforms a similarity matrix to
a dissimilarity matrix in such a way that distances between the points
returned in `Y`

approximate `sqrt(1-D)`

.
To use a different transformation, transform the similarities prior
to calling `mdscale`

.

`[Y,stress] = mdscale(D,p)`

returns the minimized
stress, i.e., the stress evaluated at `Y`

.

`[Y,stress,disparities] = mdscale(D,p)`

returns
the disparities, that is, the monotonic transformation of the dissimilarities `D`

.

`[...] = mdscale(D,p,'`

specifies
one or more optional parameter name/value pairs that control further
details of * Name*',

`value`

`mdscale`

. Specify `Name`

`Criterion`

— The goodness-of-fit criterion to minimize. This also determines the type of scaling, either non-metric or metric, that`mdscale`

performs. Choices for non-metric scaling are:`'stress'`

— Stress normalized by the sum of squares of the inter-point distances, also known as stress1. This is the default.`'sstress'`

— Squared stress, normalized with the sum of 4th powers of the inter-point distances.

Choices for metric scaling are:

`'metricstress'`

— Stress, normalized with the sum of squares of the dissimilarities.`'metricsstress'`

— Squared stress, normalized with the sum of 4th powers of the dissimilarities.`'sammon'`

— Sammon's nonlinear mapping criterion. Off-diagonal dissimilarities must be strictly positive with this criterion.`'strain'`

— A criterion equivalent to that used in classical multidimensional scaling.

`Weights`

— A matrix or vector the same size as`D`

, containing nonnegative dissimilarity weights. You can use these to weight the contribution of the corresponding elements of`D`

in computing and minimizing stress. Elements of`D`

corresponding to zero weights are effectively ignored.### Note

When you specify weights as a full matrix, its diagonal elements are ignored and have no effect, since the corresponding diagonal elements of

`D`

do not enter into the stress calculation.`Start`

— Method used to choose the initial configuration of points for Y. The choices are`'cmdscale'`

— Use the classical multidimensional scaling solution. This is the default.`'cmdscale'`

is not valid when there are zero weights.`'random'`

— Choose locations randomly from an appropriately scaled p-dimensional normal distribution with uncorrelated coordinates.An

*n*-by-`p`

matrix of initial locations, where n is the size of the matrix`D`

and`p`

is the number of columns of the output matrix`Y`

. In this case, you can pass in`[]`

for`p`

and`mdscale`

infers`p`

from the second dimension of the matrix. You can also supply a 3-D array, implying a value for`'Replicates'`

from the array's third dimension.

`Replicates`

— Number of times to repeat the scaling, each with a new initial configuration. The default is`1`

.`Options`

— Options for the iterative algorithm used to minimize the fitting criterion. Pass in an options structure created by`statset`

. For example,opts = statset(

*param1*,*val1*,*param2*,*val2*, ...); [...] = mdscale(...,'Options',opts)The choices of

`statset`

parameters are`'Display'`

— Level of display output. The choices are`'off'`

(the default),`'iter'`

, and`'final'`

.`'MaxIter'`

— Maximum number of iterations allowed. The default is`200`

.`'TolFun'`

— Termination tolerance for the stress criterion and its gradient. The default is`1e-4`

.`'TolX'`

— Termination tolerance for the configuration location step size. The default is`1e-4`

.

load cereal.mat X = [Calories Protein Fat Sodium Fiber ... Carbo Sugars Shelf Potass Vitamins]; % Take a subset from a single manufacturer. X = X(strcmp('K',cellstr(Mfg)),:); % Create a dissimilarity matrix. dissimilarities = pdist(X); % Use non-metric scaling to recreate the data in 2D, % and make a Shepard plot of the results. [Y,stress,disparities] = mdscale(dissimilarities,2); distances = pdist(Y); [dum,ord] = sortrows([disparities(:) dissimilarities(:)]); plot(dissimilarities,distances,'bo', ... dissimilarities(ord),disparities(ord),'r.-'); xlabel('Dissimilarities'); ylabel('Distances/Disparities') legend({'Distances' 'Disparities'},'Location','NW');

% Do metric scaling on the same dissimilarities. figure [Y,stress] = ... mdscale(dissimilarities,2,'criterion','metricsstress'); distances = pdist(Y); plot(dissimilarities,distances,'bo', ... [0 max(dissimilarities)],[0 max(dissimilarities)],'r.-'); xlabel('Dissimilarities'); ylabel('Distances')

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