PROC GENMOD: Log Odds Ratios and the ALR Algorithm :: SAS/STAT(R) 9.2 User's Guide, Second Edition

The GENMOD Procedure

Example 37.6 Log Odds Ratios and the ALR Algorithm

Since the respiratory data in Example 37.5 are binary, you can use the ALR algorithm to model the log odds ratios instead of using working correlations to model associations. In this example, a "fully parameterized cluster" model for the log odds ratio is fit. That is, there is a log odds ratio parameter for each unique pair of responses within clusters, and all clusters are parameterized identically. The following statements fit the same regression model for the mean as in Example 37.5 but use a regression model for the log odds ratios instead of a working correlation. The LOGOR=FULLCLUST option specifies a fully parameterized log odds ratio model.

   proc genmod data=resp descend;
      class id treatment(ref="P") center(ref="1") sex(ref="M")
         baseline(ref="0") / param=ref;
      model outcome=treatment center sex age baseline / dist=bin;
      repeated  subject=id(center) / logor=fullclust;
   run;

The results of fitting the model are displayed in Output 37.6.1 along with a table that shows the correspondence between the log odds ratio parameters and the within-cluster pairs. Model goodness-of-fit criteria are shown in Output 37.6.2. The QIC for the ALR model shown in Output 37.6.2 is 511.86, whereas the QIC for the unstructured working correlation model shown in Output 37.5.4 is 512.34, indicating that the ALR model is a slightly better fit.

Output 37.6.1 Results of Model Fitting

The GENMOD Procedure

Log Odds Ratio Parameter Information
Parameter	Group
Alpha1	(1, 2)
Alpha2	(1, 3)
Alpha3	(1, 4)
Alpha4	(2, 3)
Alpha5	(2, 4)
Alpha6	(3, 4)

Analysis Of GEE Parameter Estimates
Empirical Standard Error Estimates
Parameter		Estimate	Standard Error	95% Confidence Limits		Z	Pr > \|Z\|
Intercept		-0.9266	0.4513	-1.8111	-0.0421	-2.05	0.0400
treatment	A	1.2611	0.3406	0.5934	1.9287	3.70	0.0002
center	2	0.6287	0.3486	-0.0545	1.3119	1.80	0.0713
sex	F	0.1024	0.4362	-0.7526	0.9575	0.23	0.8144
age		-0.0162	0.0125	-0.0407	0.0084	-1.29	0.1977
baseline	1	1.8980	0.3404	1.2308	2.5652	5.58	<.0001
Alpha1		1.6109	0.4892	0.6522	2.5696	3.29	0.0010
Alpha2		1.0771	0.4834	0.1297	2.0246	2.23	0.0259
Alpha3		1.5875	0.4735	0.6594	2.5155	3.35	0.0008
Alpha4		2.1224	0.5022	1.1381	3.1068	4.23	<.0001
Alpha5		1.8818	0.4686	0.9634	2.8001	4.02	<.0001
Alpha6		2.1046	0.4949	1.1347	3.0745	4.25	<.0001

Output 37.6.2 Model Fit Criteria

GEE Fit Criteria
QIC	511.8589
QICu	499.6516

You can fit the same model by fully specifying the z-matrix. The following statements create a data set containing the full z-matrix:

   data zin;
      keep id center z1-z6 y1 y2;
      array zin(6) z1-z6;
      set resp ;
      by center id;
      if first.id
         then do;
            t = 0;
            do m = 1 to 4;
               do n = m+1 to 4;
                  do j = 1 to 6;
                     zin(j) = 0;
                  end;
               y1 = m;
               y2 = n;
               t + 1;
               zin(t) = 1;
               output;
               end;
            end;
         end;
   run;

   proc print data=zin (obs=12);

Output 37.6.3 displays the full z-matrix for the first two clusters. The z-matrix is identical for all clusters in this example.

Output 37.6.3 Full z-Matrix Data Set

Obs	z1	z2	z3	z4	z5	z6	center	id	y1	y2
1	1	0	0	0	0	0	1	1	1	2
2	0	1	0	0	0	0	1	1	1	3
3	0	0	1	0	0	0	1	1	1	4
4	0	0	0	1	0	0	1	1	2	3
5	0	0	0	0	1	0	1	1	2	4
6	0	0	0	0	0	1	1	1	3	4
7	1	0	0	0	0	0	1	2	1	2
8	0	1	0	0	0	0	1	2	1	3
9	0	0	1	0	0	0	1	2	1	4
10	0	0	0	1	0	0	1	2	2	3
11	0	0	0	0	1	0	1	2	2	4
12	0	0	0	0	0	1	1	2	3	4

The following statements fit the model for fully parameterized clusters by fully specifying the z-matrix. The results are identical to those shown previously.

   proc genmod data=resp descend;
      class id treatment(ref="P") center(ref="1") sex(ref="M")
         baseline(ref="0") / param=ref;
      model outcome=treatment center sex age baseline / dist=bin;
      repeated  subject=id(center) / logor=zfull
                                     zdata=zin
                                     zrow =(z1-z6)
                                     ypair=(y1 y2) ;
   run;

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Obs	z1	z2	z3	z4	z5	z6	center	id	y1	y2
1	1	0	0	0	0	0	1	1	1	2
2	0	1	0	0	0	0	1	1	1	3
3	0	0	1	0	0	0	1	1	1	4
4	0	0	0	1	0	0	1	1	2	3
5	0	0	0	0	1	0	1	1	2	4
6	0	0	0	0	0	1	1	1	3	4
7	1	0	0	0	0	0	1	2	1	2
8	0	1	0	0	0	0	1	2	1	3
9	0	0	1	0	0	0	1	2	1	4
10	0	0	0	1	0	0	1	2	2	3
11	0	0	0	0	1	0	1	2	2	4
12	0	0	0	0	0	1	1	2	3	4

Obs	z1	z2	z3	z4	z5	z6	center	id	y1	y2
1	1	0	0	0	0	0	1	1	1	2
2	0	1	0	0	0	0	1	1	1	3
3	0	0	1	0	0	0	1	1	1	4
4	0	0	0	1	0	0	1	1	2	3
5	0	0	0	0	1	0	1	1	2	4
6	0	0	0	0	0	1	1	1	3	4
7	1	0	0	0	0	0	1	2	1	2
8	0	1	0	0	0	0	1	2	1	3
9	0	0	1	0	0	0	1	2	1	4
10	0	0	0	1	0	0	1	2	2	3
11	0	0	0	0	1	0	1	2	2	4
12	0	0	0	0	0	1	1	2	3	4

Obs	z1	z2	z3	z4	z5	z6	center	id	y1	y2
1	1	0	0	0	0	0	1	1	1	2
2	0	1	0	0	0	0	1	1	1	3
3	0	0	1	0	0	0	1	1	1	4
4	0	0	0	1	0	0	1	1	2	3
5	0	0	0	0	1	0	1	1	2	4
6	0	0	0	0	0	1	1	1	3	4
7	1	0	0	0	0	0	1	2	1	2
8	0	1	0	0	0	0	1	2	1	3
9	0	0	1	0	0	0	1	2	1	4
10	0	0	0	1	0	0	1	2	2	3
11	0	0	0	0	1	0	1	2	2	4
12	0	0	0	0	0	1	1	2	3	4