The HPGENSELECT Procedure

Example 8.1 Model Selection

The following HPGENSELECT statements examine the same data that is used in the section Getting Started: HPGENSELECT Procedure, but they request model selection via the forward selection technique. Model effects are added in the order of their significance until no more effects make a significant improvement of the current model. The DETAILS=ALL option in the SELECTION statement requests that all tables that are related to model selection be produced.

The data set getStarted is shown in the section Getting Started: HPGENSELECT Procedure. It contains 100 observations on a count response variable (Y), a continuous variable (Total) to be used in Example 8.3, and five categorical variables (C1C5), each of which has four numerical levels.

A log-linked Poisson regression model is specified by using classification effects for variables C1C5. The following statements request model selection by using the forward selection method:

proc hpgenselect data=getStarted;
   class C1-C5;
   model Y = C1-C5 / Distribution=Poisson;
   selection method=forward details=all;
run;

The model selection tables are shown in Output 8.1.1 through Output 8.1.3.

The "Selection Information" table in Output 8.1.1 summarizes the settings for the model selection. Effects are added to the model only if they produce a significant improvement as judged by comparing the p-value of a score test to the entry significance level (SLE), which is 0.05 by default. The forward selection stops when no effect outside the model meets this criterion.

Output 8.1.1: Selection Information

The HPGENSELECT Procedure

Selection Information
Selection Method Forward
Select Criterion Significance Level
Stop Criterion Significance Level
Effect Hierarchy Enforced None
Entry Significance Level (SLE) 0.05
Stop Horizon 1



The "Selection Summary" table in Output 8.1.2 shows the effects that were added to the model and their significance level. Step 0 refers to the null model that contains only an intercept. In the next step, effect C2 made the most significant contribution to the model among the candidate effects (p < 0.0001). In step 2, the most significant contribution when adding an effect to a model that contains the intercept and C2 was made by C5. In step 3, the variable C1 (p = 0.0496) was added. In the subsequent step, no effect could be added to the model that would produce a p-value less than 0.05, so variable selection stops.

Output 8.1.2: Selection Summary Information

The HPGENSELECT Procedure

Selection Summary
Step Effect
Entered
Number
Effects In
p Value
0 Intercept 1 .
1 C2 2 <.0001
2 C5 3 <.0001
3 C1 4 0.0496

Selection stopped because no candidate for entry is significant at the 0.05 level.

Selected Effects: Intercept C1 C2 C5



The DETAILS=ALL option produces the "Selection Details" table, which provides fit statistics and the value of the score test chi-square statistic at each step.

Output 8.1.3: Selection Details

Selection Details
Step Description Effects
In Model
Chi-Square Pr > ChiSq -2 LogL AIC AICC BIC
0 Initial Model 1     350.193 352.193 352.234 354.798
1 C2 entered 2 25.7340 <.0001 324.611 332.611 333.032 343.032
2 C5 entered 3 23.0291 <.0001 303.580 317.580 318.798 335.817
3 C1 entered 4 7.8328 0.0496 295.263 315.263 317.735 341.315



Output 8.1.4 displays information about the selected model. Notice that the –2 log likelihood value in the "Fit Statistics" table is larger than the value for the full model in Figure 8.7. This is expected because the selected model contains only a subset of the parameters. Because the selected model is more parsimonious than the full model, the information criteria AIC, AICC and BIC are smaller than in the full model, indicating a better fit.

Output 8.1.4: Fit Statistics

Fit Statistics
-2 Log Likelihood 295.26
AIC (smaller is better) 315.26
AICC (smaller is better) 317.74
BIC (smaller is better) 341.31
Pearson Chi-Square 85.0656
Pearson Chi-Square/DF 0.9452



The parameter estimates of the selected model are given in Output 8.1.5. Notice that the effects are listed in the "Parameter Estimates" table in the order in which they were specified in the MODEL statement and not in the order in which they were added to the model.

Output 8.1.5: Parameter Estimates

Parameter Estimates
Parameter DF Estimate Standard
Error
Chi-Square Pr > ChiSq
Intercept 1 0.775498 0.242561 10.2216 0.0014
C1 0 1 -0.211240 0.207209 1.0393 0.3080
C1 1 1 -0.685575 0.255713 7.1879 0.0073
C1 2 1 -0.127612 0.203663 0.3926 0.5309
C1 3 0 0 . . .
C2 0 1 0.958378 0.239731 15.9817 <.0001
C2 1 1 0.738529 0.237098 9.7024 0.0018
C2 2 1 0.211075 0.255791 0.6809 0.4093
C2 3 0 0 . . .
C5 0 1 -0.825545 0.214054 14.8743 0.0001
C5 1 1 -0.697611 0.202607 11.8555 0.0006
C5 2 1 -0.566706 0.213961 7.0153 0.0081
C5 3 0 0 . . .