The ENTROPY Procedure(Experimental)

Consider the following test score data compiled by Coleman et al. (1966):

title "Test Scores Compiled by Coleman et al. (1966)";
data coleman;
   input test_score 6.2 teach_sal 6.2 prcnt_prof 8.2
            socio_stat 9.2 teach_score 8.2 mom_ed 7.2;
   label test_score="Average sixth grade test scores in observed district";
   label teach_sal="Average teacher salaries per student (1000s of dollars)";
   label prcnt_prof="Percent of students' fathers with professional employment";
   label socio_stat="Composite measure of socio-economic status in the district";
   label teach_score="Average verbal score for teachers";
   label mom_ed="Average level of education (years) of the students' mothers";
datalines;
37.01   3.83   28.87     7.20   26.60   6.19

   ... more lines ...   

This data set contains outliers, and the condition number of the matrix of regressors, , is large, which indicates collinearity among the regressors. Since the maximum entropy estimates are both robust with respect to the outliers and also less sensitive to a high condition number of the matrix, maximum entropy estimation is a good choice for this problem.

To fit a simple linear model to these data by using PROC ENTROPY, use the following statements:

proc entropy data=coleman;
   model test_score = teach_sal prcnt_prof socio_stat teach_score mom_ed;
run;

This requests the estimation of a linear model for TEST_SCORE with the following form:

This estimation produces the "Model Summary" table in Figure 2, which shows the equation variables used in the estimation.

Since support points and prior weights are not specified in this example, they are not shown in the "Model Summary" table. The next four pieces of information displayed in Figure 3 are the "Data Set Options," the "Minimization Summary," the "Final Information Measures," and the "Observations Processed."

The item labeled "Objective Function Value" is the value of the entropy estimation criterion for this estimation problem. This measure is analogous to the log-likelihood value in a maximum likelihood estimation. The "Parameter Information Index" and the "Error Information Index" are normalized entropy values that measure the proximity of the solution to the prior or target distributions.

The next table displayed is the ANOVA table, shown in Figure 4. This is in the same form as the ANOVA table for the MODEL procedure, since this is also a multivariate procedure.

The last table displayed is the "Parameter Estimates" table, shown in Figure 5. The difference between this parameter estimates table and the parameter estimates table produced by other regression procedures is that the standard error and the probabilities are labeled as approximate.

The parameter estimates produced by the REG procedure for this same model are shown in Figure 6. Note that the parameters and standard errors from PROC REG are much different than estimates produced by PROC ENTROPY.

proc reg data=coleman plots=residualchart(unpack);
   model test_score = teach_sal prcnt_prof socio_stat teach_score mom_ed;
run;

This data set contains two outliers, observations 3 and 18. These can be seen in a plot of the residuals shown in Figure 7: the studentized residuals of observations 3 and 18 are outside , the confidence interval for a Student’s t distribution with 13 degrees of freedom.

Figure 7: PROC REG Residuals with Outliers

The presence of outliers suggests that a robust estimator such as M-estimator in the ROBUSTREG procedure should be used. The following statements use the ROBUSTREG procedure to estimate the model:

proc robustreg data=coleman;
   model test_score = teach_sal prcnt_prof
                      socio_stat teach_score mom_ed;
run;

The results of the estimation are shown in Figure 8.

Note that TEACH_SAL(VAR1) and MOM_ED(VAR5) change greatly when the robust estimation is used. Unfortunately, these two coefficients are negative, which implies that the test scores increase with decreasing teacher salaries and decreasing levels of the mother’s education. Since ROBUSTREG is robust to outliers, they are not causing the counterintuitive parameter estimates.

The condition number of the regressor matrix also plays a important role in parameter estimation. The condition number of the matrix can be obtained by specifying the COLLIN option in the PROC ENTROPY statement.

proc entropy data=coleman collin;
   model test_score = teach_sal prcnt_prof socio_stat teach_score mom_ed;
run;

The output produced by the COLLIN option is shown in Figure 9.

The condition number of the matrix is reported to be 84.85. This means that the condition number of is , which is very large.

Ridge regression can be used to offset some of the problems associated with ill-conditioned matrices. Using the formula for the ridge value as

where and are the least squares estimators of and and . A ridge regression of the test score model was performed by using the data set with the outliers removed. The following PROC REG code performs the ridge regression:

data coleman;
   set coleman;
   if _n_ = 3 or _n_ = 18 then delete;
run;

proc reg data=coleman ridge=0.9 outest=t noprint;
   model test_score = teach_sal prcnt_prof socio_stat teach_score mom_ed;
run;

proc print data=t;
run;

The results of the estimation are shown in Figure 10.

Note that the ridge regression estimates are much closer to the estimates produced by the ENTROPY procedure that uses the original data set. Ridge regressions are not robust to outliers as maximum entropy estimates are. This might explain why the estimates still differ for TEACH_SAL.