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cancor

Performs canonical correlation analysis between two sets of variables.

Prototype

	function cancor (
		x [*][*] : numeric,  
		y [*[[*] : numeric,  
		option   : logical   
	)

	return_val [*] :  float or double

Arguments

An array of two dimensions of size (NX,NOBS) where NOBS represents the number of observations and NX represents the number of independent variables. The x are the (linearly independent) predictor variables.

An array of two dimensions of size (NY,NOBS) where NOBS represents the number of observations and NY represents the number of dependent variables. The y are the (dependent) predictand variables. Note: NY.le.NX.

option

A logical variable. Currently not used.

Return value

An array containing the canonical correlation coefficients. The return values will be of type double if x or y is double, and float otherwise.

A suite of attributes is returned:

          (a) ndof:  degrees of freedom (one dimensional array)
          (b) chisq: chi-squares values (one dimensional array)
          (c) wlam : Wilk's Lambda (one dimensional array)
          (d) coefx: two dimensional array (NY,NX) containing the 
                     right hand [x] canonical loading coefficients.
          (f) coefy: two dimensional array (NY,NY) containing the 
                     left hand [y] canonical loading coefficients.

ndof will be type integer. chisq, coefr and coefl will be of the same type as the returned canonical correlations.

Description

PLEASE NOTE: Cherry (1996) discusses Singular Value Decomposition (SVD) and Canonical Correlation Analysis (CCA). Cherry's summary comment is: "Both methods have a high potential to produce spurious spatial patterns. Caution is always called for in interpreting results from either method." Newman and Sardeshmukh (1995) came to a similar conclusion in their paper which focused on SVD: "These results suggest that any physical interpretation of SVD pairs may be unjustified."


REFERENCES:

Cherry, S. (1996):  Singular Value Decomposition Analysis and Canonical Correlation Analysis. 
J. Climate: pp 2003-2009.

Newman, M. and Sardeshmukh, P.D. (1995): A Caveat Concerning Singular Value Decomposition 
J. Climate: pp 352-360.

------------------------------------------------------------

cancor performs canonical correlation analysis (CCA). Canonical correlation explores the relationships between standardized variables. The objectives are similar to multiple linear regression except there are multiple y variables (i.e., determine linear combinations of the y variables which are well explained by linear combinations of the x variables). It can be used as either a hypothesis testing or an exploratory method.

It may be possible to ascribe physical interpretation to the results. However, like EOF analysis, the mathematical manipulations are not based on physical equations.

The y are the dependent variables and x are the predictor variables. The ndof and chisq information can be used to test the null hypothesis that all canonical correlations are zero. NCL's gammainc can be used to determine the significance level. See the examples.

The eigenvalues may be determined by squaring returned canonical correlations.

The canonical scores can be derived by multiplying the standardized original variables by the canonical loading vectors coefx and coefy.

NCL uses "SUBROUTINE CANOR" from the old IBM "Scientific Subroutine Package" (SSP) to perform the calculations.


       http://www.decuslib.com/decus/vax87d/rcaf87/ssp/ssp.for

NOTE: Some users have reported obvious errors when using many more variables than observations. Although the original documentation for the subroutine does not contain any comments regarding this situation, it seems that the number of observations should be (?much?) larger than the combined number of variables.

Examples

Data Source:

  Statistics and Data Analysis in Geology
  John C Davis
  John Wiley and Sons: 2002:   3rd Edition

Example 1

This is the "well" data from the above source [Table 6-38, p583]. The y are the core measurements and the x are the log measurements.

begin
  y = (/  (/  3.1,  3.4,  3.4,  2.8,  2.5,  2.3,  2.3,  2.6 \  
           ,  6.0,  5.2,  3.9,  4.7,  5.1,  5.0,  6.1/)     \
       ,  (/ 64.0, 69.0, 65.0, 62.0, 56.0, 56.0, 54.0, 60.0 \
           , 97.0, 67.0, 82.0, 80.0, 77.0, 79.0, 81.0/)     \
       ,  (/ 28.8, 25.1, 38.0, 15.1, 58.9, 61.7,129.0,110.0 \
           ,  5.2, 18.2, 26.9, 12.9, 12.0, 11.0, 70.8/)     /)

  x = (/  (/  0.1,  0.4,  0.1,  0.4,  0.1,  0.3,  0.2,  1.6 \  
           ,  0.0, 18.0,  6.5,  2.5,  0.1,  0.0,  0.0/)     \
       ,  (/  3.9,  7.0,  6.1,  6.2,  5.9,  4.7,  6.2, 12.7 \
           ,  3.0, 18.9, 18.4, 17.9, 12.3, 10.4,  5.2/)     \ 
       ,  (/ 28.2, 17.2, 24.6, 19.3, 15.3, 14.9, 29.0, 26.7 \
           ,  0.0, 26.4, 19.0, 16.8, 11.7,  0.0,  0.0/)     \ 
       ,  (/ 53.8, 55.6, 54.2, 63.0, 73.0, 61.6, 37.1, 34.6 \
           , 96.6, 32.3, 48.3, 48.0, 72.6, 91.4, 97.8/)     /)

         ; the following is for informational purposes only

  dimy   = dimsizes(y)
  dimx   = dimsizes(x)

  my     = dimy(0)   ; Y
  mx     = dimx(0)   ; X
  nobs   = dimx(1)   
   
         ; canonical correlation

  opt    = False
  canr   = cancor(x, y, opt)

  prob   = gammainc( 0.5*canr@chisq,  0.5*canr@ndof ) 

  print(canr)
  print(canr@ndof)
  print(canr@chisq)
  print(canr@wlam)
  print(canr@coefx)
  print(canr@coefy)

  print(prob)
end

The (edited) output from the print follows:

Variable: canr
Type: float
Dimensions and sizes:   [4]

(0)     0.8672 
(1)     0.5953 
(2)     0.2670

Variable: ndof
Type: integer
Dimensions and sizes:   [3]

(0)     12
(1)     6
(2)     2

Variable: chisq
Type: float
Number of Dimensions: 1

(0)     20.96687
(1)      5.62574
(2)      0.81361

Variable: wlam
Type: float
Dimensions and sizes:   [3]

(0)     0.14866
(1)     0.59963
(2)     0.92870


Variable: coefx
Type: float
Dimensions and sizes:   [3] x [4]

(0,0)   -0.38379
(0,1)   -0.32543
(0,2)   -0.11506
(0,3)   -0.85647

(1,0)    0.34417
(1,1)   -0.04713
(1,2)    0.67963
(1,3)    0.64607

(2,0)   -0.15488
(2,1)    0.33424
(2,2)    0.63058
(2,3)    0.68312


Variable: coefy
Type: float
Dimensions and sizes:   [3] x [3]

(0,0)   -0.69702
(0,1)    0.16100
(0,2)    0.17747

(1,0)    0.42498
(1,1)   -0.68048
(1,2)   -0.21716

(2,0)   -0.21955
(2,1)    0.10104
(2,2)   -0.19530


Variable: prob
Type: float
Dimensions and sizes:   [3]
(0)     0.949132
(1)     0.533608
(2)     0.334226

Example 2

Data Values (BOXES.TXT) from the above reference: There are 7 variables (columns) and 25 rows (observations).

  3.760  3.660  0.540  5.275  9.768 13.741  4.782
  8.590  4.990  1.340 10.022  7.500 10.162  2.130
  6.220  6.140  4.520  9.842  2.175  2.732  1.089
  7.570  7.280  7.070 12.662  1.791  2.101  0.822
  9.030  7.080  2.590 11.762  4.539  6.217  1.276
  5.510  3.980  1.300  6.924  5.326  7.304  2.403
  3.270  0.620  0.440  3.357  7.629  8.838  8.389
  8.740  7.000  3.310 11.675  3.529  4.757  1.119
  9.640  9.490  1.030 13.567 13.133 18.519  2.354
  9.730  1.330  1.000  9.871  9.871 11.064  3.704
  8.590  2.980  1.170  9.170  7.851  9.909  2.616
  7.120  5.490  3.680  9.716  2.642  3.430  1.189
  4.690  3.010  2.170  5.983  2.760  3.554  2.013
  5.510  1.340  1.270  5.808  4.566  5.382  3.427
  1.660  1.610  1.570  2.799  1.783  2.087  3.716
  5.900  5.760  1.550  8.388  5.395  7.497  1.973
  9.840  9.270  1.510 13.604  9.017 12.668  1.745
  8.390  4.920  2.540 10.053  3.956  5.237  1.432
  4.940  4.380  1.030  6.678  6.494  9.059  2.807
  7.230  2.300  1.770  7.790  4.393  5.374  2.274
  9.460  7.310  1.040 11.999 11.579 16.182  2.415
  9.550  5.350  4.250 11.742  2.766  3.509  1.054
  4.940  4.520  4.500  8.067  1.793  2.103  1.292
  8.210  3.080  2.420  9.097  3.753  4.657  1.719
  9.410  6.440  5.110 12.495  2.446  3.103  0.914

Use asciiread to read the BOXES.TXT ascii file with no "header" information.

The above book uses the first three columns as the dependent y variables and the last 4 columns as the predictor x variables. The input array must be partitioned using NCL's array syntax and dimension reordering. The example will explicitly create x and y variables for clarity.

       diri    = "./"
       fili    = "BOXES.TXT"
                                         ; read the entire data array
       nvar    = 7                       ; total number of variables
       nrow    = 25                      ; number of observations
       data    = asciiread( diri+fili, (/nrow,ncol/), "float")
       data!0  = "row"                   ; name the dimensions
       data!1  = "col"
                                         ; match book subsetting
       my      = 3                       ; y
       mx      = 4                       ; x
   
                                         ; reorder via named dimensions [3 x 25]
                                         ; subset via array syntax      [4 x 25]
       y       = data(col|0:my-1,row|:)  ; [var | 3] x [row | 25]
       x       = data(col|my:   ,row|:)  ; [var | 4] x [row | 25]

       canr    = cancor( x, y, False)

       prob    = gammainc( 0.5*canr@chisq,  0.5*canr@ndof )

       print(canr)
       print(canr@ndof)
       print(canr@chisq)
       print(canr@wlam)
       print(canr@coefx)
       print(canr@coefy)

       print(prob)