SAS Data Analysis Examples
Canonical Correlation Analysis

Examples of Canonical Correlation Analysis

Version info: Code for this page was tested in SAS 9.3.

Canonical correlation analysis is used to identify and measure the associations among two sets of variables.  Canonical correlation is appropriate in the same situations where multiple regression would be, but where are there are multiple intercorrelated outcome variables. Canonical correlation analysis determines a set of canonical variates, orthogonal linear combinations of the variables within each set that best explain the variability both within and between sets. 

Please Note: The purpose of this page is to show how to use various data analysis commands. It does not cover all aspects of the research process which researchers are expected to do. In particular, it does not cover data cleaning and checking, verification of assumptions, model diagnostics and potential follow-up analyses.

Examples of canonical correlation analysis

Example 1. A researcher has collected data on three psychological variables, four academic variables (standardized test scores) and gender for 600 college freshman. She is interested in how the set of psychological variables relates to the academic variables and gender. In particular, the researcher is interested in how many dimensions (canonical variables) are necessary to understand the association between the two sets of variables.

Example 2. A researcher is interested in exploring associations among factors from two multidimensional personality tests, the MMPI and the NEO. She is interested in what dimensions are common between the tests and how much variance is shared between them. She is specifically interested in finding whether the neuroticism dimension from the NEO can account for a substantial amount of shared variance between the two tests.

Description of the Data

Let's pursue Example 1 from above.

We have included the data file, which can be obtained by clicking on mmreg.sas7bdat. The dataset has 600 observations on eight variables. The psychological variables are locus of control, self-concept and motivation. The academic variables are standardized tests in reading, writing, math and science. Additionally, the variable female is a zero-one indicator variable with the one indicating a female student.

Let's look at the data.


proc means data=mylib.mmreg;
run;

                                       The MEANS Procedure

 Variable          Label               N          Mean       Std Dev       Minimum       Maximum
 -----------------------------------------------------------------------------------------------
 ID                                  600   300.5000000   173.3493582     1.0000000   600.0000000
 LOCUS_OF_CONTROL  locus of control  600     0.0965333     0.6702799    -2.2300000     1.3600000
 SELF_CONCEPT      self-concept      600     0.0049167     0.7055125    -2.6199999     1.1900001
 MOTIVATION        motivation        600     0.6608333     0.3427294             0     1.0000000
 READ              reading score     600    51.9018334    10.1029830    28.2999992    76.0000000
 WRITE             writing score     600    52.3848333     9.7264550    25.5000000    67.0999985
 MATH              math score        600    51.8490000     9.4147363    31.7999992    75.5000000
 SCIENCE           science score     600    51.7633332     9.7061789    26.0000000    74.1999969
 FEMALE                              600     0.5450000     0.4983864             0     1.0000000
 -----------------------------------------------------------------------------------------------


proc freq data=mylib.mmreg;
  table female;
run;

                                        The FREQ Procedure

                                                      Cumulative    Cumulative
                   FEMALE    Frequency     Percent     Frequency      Percent
                   -----------------------------------------------------------
                        0         273       45.50           273        45.50
                        1         327       54.50           600       100.00

We did not include correlations among the variables at this point because we will get them later as part of the canonical correlation analysis.

Analysis methods you might consider

Before we show how you can analyze this with a canonical correlation analysis, let's consider some other methods that you might use.

Canonical correlation analysis

Due to the length of the output, we will be making comments in several places along the way.

proc cancorr corr data=mylib.mmreg;
  var locus_of_control self_concept motivation;
  with read write math science female;
run;

The corr option on the proc cancorr statement produces correlations within and between the two sets of variables are given below.

                                      The CANCORR Procedure

                            Correlations Among the Original Variables

                               Correlations Among the VAR Variables

                                       LOCUS_OF_
                                         CONTROL      SELF_CONCEPT        MOTIVATION

              LOCUS_OF_CONTROL            1.0000            0.1712            0.2451
              SELF_CONCEPT                0.1712            1.0000            0.2886
              MOTIVATION                  0.2451            0.2886            1.0000

                              Correlations Among the WITH Variables

                     READ             WRITE              MATH           SCIENCE            FEMALE

READ               1.0000            0.6286            0.6793            0.6907           -0.0417
WRITE              0.6286            1.0000            0.6327            0.5691            0.2443
MATH               0.6793            0.6327            1.0000            0.6495           -0.0482
SCIENCE            0.6907            0.5691            0.6495            1.0000           -0.1382
FEMALE            -0.0417            0.2443           -0.0482           -0.1382            1.0000

                  Correlations Between the VAR Variables and the WITH Variables

                             READ           WRITE            MATH         SCIENCE          FEMALE

 LOCUS_OF_CONTROL          0.3736          0.3589          0.3373          0.3246          0.1134
 SELF_CONCEPT              0.0607          0.0194          0.0536          0.0698         -0.1260
 MOTIVATION                0.2106          0.2542          0.1950          0.1157          0.0981

                                          The SAS System          10:43 Tuesday,

The output below gives the three canonical correlations and the multivariate tests of the dimensions. These results show that the first two of the three canonical correlations are statistically significant. The output also includes the four multivariate criteria and the F approximations.

                                      The CANCORR Procedure

                                  Canonical Correlation Analysis

                                           Adjusted    Approximate        Squared
                           Canonical      Canonical       Standard      Canonical
                         Correlation    Correlation          Error    Correlation

                       1    0.464086       0.455474       0.032059       0.215376
                       2    0.167509        .             0.039712       0.028059
                       3    0.103991        .             0.040417       0.010814

                                                        Test of H0: The canonical correlations in
                    Eigenvalues of Inv(E)*H           the current row and all that follow are zero
                      = CanRsq/(1-CanRsq)
                                                      Likelihood Approximate
          Eigenvalue Difference Proportion Cumulative      Ratio     F Value Num DF Den DF Pr > F

        1     0.2745     0.2456     0.8734     0.8734 0.75436113       11.72     15 1634.7 <.0001
        2     0.0289     0.0179     0.0919     0.9652 0.96142996        2.94      8   1186 0.0029
        3     0.0109                0.0348     1.0000 0.98918584        2.16      3    594 0.0911



                          Multivariate Statistics and F Approximations

                                      S=3    M=0.5    N=295

         Statistic                        Value    F Value    Num DF    Den DF    Pr > F

         Wilks' Lambda               0.75436113      11.72        15    1634.7    <.0001
         Pillai's Trace              0.25424936      11.00        15      1782    <.0001
         Hotelling-Lawley Trace      0.31429738      12.38        15      1113    <.0001
         Roy's Greatest Root         0.27449563      32.61         5       594    <.0001

                  NOTE: F Statistic for Roy's Greatest Root is an upper bound.

In general, the number of canonical dimensions is equal to the number of variables in the smaller set; however, the number of significant dimensions may be even smaller. Canonical dimensions, also known as canonical variates, are similar to latent variables that are found in factor analysis, except that canonical variates also maximize the correlation between the two sets of variables.  For this particular model there are three canonical dimensions of which only the first two are statistically significant. The first test of dimensions tests whether all three dimensions are significant (F = 11.72), the next test tests whether dimensions 2 and 3 combined are significant (F = 2.94). Finally, the last test tests whether dimension 3, by itself, is significant (F = 2.16). Therefore dimensions 1 and 2 are each significant while the third dimension is not.

Next, the raw canonical coefficients are shown below. The raw canonical coefficients are interpreted in a manner analogous to interpreting regression coefficients i.e., for the variable read, a one unit increase in reading leads to a .0446 increase in the first canonical variate of set 2 when all of the other variables are held constant. Here is another example: being female leads to a .6321 increase in dimension 1 for set 2 with the other predictors held constant.

                         Raw Canonical Coefficients for the VAR Variables

                                                         V1                V2                V3

   LOCUS_OF_CONTROL      locus of control      1.2538339076      0.6214775237      -0.661689607
   SELF_CONCEPT          self-concept           -0.35134993      1.1876866562      0.8267209411
   MOTIVATION            motivation            1.2624203286      -2.027264053      2.0002284379

                        Raw Canonical Coefficients for the WITH Variables

                                                   W1                W2                W3

         READ         reading score      0.0446205959      0.0049100176      0.0213805581
         WRITE        writing score      0.0358771125      -0.042071471      0.0913073288
         MATH         math score         0.0234171847      -0.004229472      0.0093982096
         SCIENCE      science score      0.0050251567      0.0851621751      -0.109835018
         FEMALE                          0.6321192387      -1.084642482      -1.794646917

The raw coefficients are followed by the standardized canonical coefficients shown below.  When the variables in the model have very different standard deviations, the standardized coefficients allow for easier comparisons among the variables.  The standardized canonical coefficients are interpreted in a manner analogous to interpreting standardized regression coefficients. For example, consider the variable read, a one standard deviation increase in reading leads to a 0.45 standard deviation increase in the score on the first canonical variate for set 2 when the other variables in the model are held constant.

                    Standardized Canonical Coefficients for the VAR Variables

                                                           V1            V2            V3

         LOCUS_OF_CONTROL      locus of control        0.8404        0.4166       -0.4435
         SELF_CONCEPT          self-concept           -0.2479        0.8379        0.5833
         MOTIVATION            motivation              0.4327       -0.6948        0.6855

                    Standardized Canonical Coefficients for the WITH Variables

                                                     W1            W2            W3

               READ         reading score        0.4508        0.0496        0.2160
               WRITE        writing score        0.3490       -0.4092        0.8881
               MATH         math score           0.2205       -0.0398        0.0885
               SCIENCE      science score        0.0488        0.8266       -1.0661
               FEMALE                            0.3150       -0.5406       -0.8944

Below are correlations between observed variables and canonical variables which are known as the canonical loadings, which SAS labels as the canonical structure.

                                       Canonical Structure

               Correlations Between the VAR Variables and Their Canonical Variables

                                                           V1            V2            V3

         LOCUS_OF_CONTROL      locus of control        0.9040        0.3897       -0.1756
         SELF_CONCEPT          self-concept            0.0208        0.7087        0.7052
         MOTIVATION            motivation              0.5672       -0.3509        0.7451

               Correlations Between the WITH Variables and Their Canonical Variables

                                                     W1            W2            W3

               READ         reading score        0.8404        0.3588        0.1354
               WRITE        writing score        0.8765       -0.0648        0.2546
               MATH         math score           0.7639        0.2979        0.1478
               SCIENCE      science score        0.6584        0.6768       -0.2304
               FEMALE                            0.3641       -0.7549       -0.5434

     Correlations Between the VAR Variables and the Canonical Variables of the WITH Variables

                                                           W1            W2            W3

         LOCUS_OF_CONTROL      locus of control        0.4196        0.0653       -0.0183
         SELF_CONCEPT          self-concept            0.0097        0.1187        0.0733
         MOTIVATION            motivation              0.2632       -0.0588        0.0775

     Correlations Between the WITH Variables and the Canonical Variables of the VAR Variables

                                                     V1            V2            V3

               READ         reading score        0.3900        0.0601        0.0141
               WRITE        writing score        0.4068       -0.0109        0.0265
               MATH         math score           0.3545        0.0499        0.0154
               SCIENCE      science score        0.3056        0.1134       -0.0240
               FEMALE                            0.1690       -0.1265       -0.0565

Things to consider

As in the case of multivariate regression, MANOVA and so on, for valid inference, canonical correlation analysis requires the multivariate normal and homogeneity of variance assumption. Canonical correlation analysis assumes a linear relationship between the canonical variates and each set of variables. Similar to multivariate regression, canonical correlation analysis requires a large sample size.

See Also

References

 

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