The CATMOD Procedure

In this multiple-population repeated measures example, from Guthrie (1981), subjects from three groups have their responses (0 or 1) recorded in each of four trials. The analysis of the marginal probabilities is directed at assessing the main effects of the repeated measurement factor (Trial) and the independent variable (Group), as well as their interaction. Although the contingency table is incomplete (only 13 of the 16 possible responses are observed), this poses no problem in the computation of the marginal probabilities. The following statements produce Output 32.6.1:

data group;
   input a b c d Group wt @@;
   datalines;
1 1 1 1 2 2     0 0 0 0 2 2     0 0 1 0 1 2     0 0 1 0 2 2
0 0 0 1 1 4     0 0 0 1 2 1     0 0 0 1 3 3     1 0 0 1 2 1
0 0 1 1 1 1     0 0 1 1 2 2     0 0 1 1 3 5     0 1 0 0 1 4
0 1 0 0 2 1     0 1 0 1 2 1     0 1 0 1 3 2     0 1 1 0 3 1
1 0 0 0 1 3     1 0 0 0 2 1     0 1 1 1 2 1     0 1 1 1 3 2
1 0 1 0 1 1     1 0 1 1 2 1     1 0 1 1 3 2  
;

title 'Multiple-Population Repeated Measures';
proc catmod data=group;
   weight wt;
   response marginals;
   model a*b*c*d=Group _response_ Group*_response_
         / freq;
   repeated Trial 4;
   title2 'Saturated Model';
run;

The analysis of variance table in Output 32.6.1 shows that there is a significant interaction between the independent variable Group and the repeated measurement factor Trial. An intermediate model (not shown) is fit in which the effects Trial and Group* Trial are replaced by Trial(Group=1), Trial(Group=2), and Trial(Group=3). Of these three effects, only the last is significant, so it is retained in the final model. The following statements produce Output 32.6.2 and Output 32.6.3:

   model a*b*c*d=Group _response_(Group=3)
         / noprofile noparm design;
   title2 'Trial Nested within Group 3';
quit;

Output 32.6.2 displays the design matrix resulting from retaining the nested effect.

The residual goodness-of-fit statistic tests the joint effect of Trial(Group=1) and Trial(Group=2). The analysis of variance table in Output 32.6.3 shows that the final model fits, that there is a significant Group effect, and that there is a significant Trial effect in Group 3.