topic Re: random statement in Statistical Procedures

random statement

mona4u — Thu, 25 Aug 2022 14:26:24 GMT

/*I'm creating this model and I have a question about the random statemnt if I'm comparing left eye to rigt ey is this random statement should be
good or should I create differnt random statemnt in differnt model to serve that porpuse */
*ex: random visit/ residual subject=usubjid type=cs;

proc glimmix data=analysisplots=studentpanel;
class sex usubjid eye visit ;
model aval= visit|eye age sex / ddfm=kr s;
random intercept / subject=usubjid ;
random eye/ residual subject=usubjid (visit ) type=cs;
lsmeans eye*visit / alpha=0.05 cl ;
/*comapre visit to their baseline */
LSMeans eye*visit/ slicediff=eye slicedifftype=control( 'Left' 'Baseline') alpha=0.05 cl ;
/*compare right eye and left eye at diffrent visits */
LSMeans eye*visit/ slicediff=visit slicedifftype=control( 'Left' 'Baseline') alpha=0.05 cl ;
run;

Re: random statement

Rick_SAS — Fri, 26 Aug 2022 14:09:28 GMT

I don't think most people would classify Eye=('Left' 'Right') as a random effect. It seems like a fixed effect because there are only two possibilities and we want to make predictions and comparisons between these levels.

A random effect would be something like a hospital or a doctor. A random effect indicates that there are many possible levels that are not present in the sample. We try to make inferences based on the levels that we have observed.

If you think that Eye should be a random effect, please describe the experiment that produced the data.

Re: random statement

StatsMan — Fri, 26 Aug 2022 15:24:06 GMT

Your RANDOM statements in your GLIMMIX code make your subject variable a random effect. That is probably what you want. @Rick_SAS is correct - eye would not usually be a random effect since it has 2 levels of interest and you have not taken a random sample of levels of eye. You supposedly have a random sample of subjects, though.

Your first RANDOM statement:

random intercept / subject=usubjid;

correlates all the measurements taken from the same level of USUBJID.

The second RANDOM statement might need some tweaking. Are you attempting to further correlate the observations from each eye within a subject over time? If so, try:

random visit / residual subject=eye(usubjid) type=ar(1);

That RANDOM statement will correlate the observations from the same eye within a subject using an AR(1) structure. You can try TYPE=CS, but you may have trouble fitting that compound symmetry structure on top of the CS structure you have implied with the first RANDOM statement (the default G-side structure gives you a common correlation among the observations from the same subject).

Re: random statement

SteveDenham — Tue, 06 Sep 2022 13:15:34 GMT

Given all that has been presented by @StatsMan and @Rick_SAS, I believe that you can model Left Eye and Right Eye as repeated measures on the individual. In this case 'eye' is a fixed effect just as 'visit' is, but you have a chance to capture within subject variation for both of these. Here is the code you propose:

proc glimmix data=analysis plots=studentpanel;
class sex usubjid eye visit ;
model aval= visit|eye age sex / ddfm=kr s;
random intercept / subject=usubjid ;
random eye/ residual subject=usubjid (visit ) type=cs;
lsmeans eye*visit / alpha=0.05 cl ;
/*comapre visit to their baseline */
LSMeans eye*visit/ slicediff=eye slicedifftype=control( 'Left' 'Baseline') alpha=0.05 cl ;
/*compare right eye and left eye at diffrent visits */
LSMeans eye*visit/ slicediff=visit slicedifftype=control( 'Left' 'Baseline') alpha=0.05 cl ;
run;

Adding one more RANDOM residual statement would accomplish what I think you want:

proc glimmix data=analysisplots=studentpanel;
class sex usubjid eye visit ;
model aval= visit|eye age sex / ddfm=kr s;
random intercept / subject=usubjid ;
RANDOM VISIT/RESIDUAL SUBJECT=USUBJID TYPE=AR(1);
random eye/ residual subject=usubjid (visit ) type=cs;
lsmeans eye*visit / alpha=0.05 cl ;
/*comapre visit to their baseline */
LSMeans eye*visit/ slicediff=eye slicedifftype=control( 'Left' 'Baseline') alpha=0.05 cl ;
/*compare right eye and left eye at diffrent visits */
LSMeans eye*visit/ slicediff=visit slicedifftype=control( 'Left' 'Baseline') alpha=0.05 cl ;
run;

I will say this though. We have had more success treating eye as a fixed effect (as pointed out by @StatsMan and @Rick_SAS ), which avoids a LOT of failure to converge and G matrix non-positive definite issues. Something like this:

proc glimmix data=ansdata_&n.  ;     
            nloptions  maxiter = 5000 ;
            class &grp_no. &time. &animal. eyen;
            model &value. = &grp_no.|&time.|eyen / ddfm=BW;
            random &time. / type = &cov. subject=eyen(&animal.);
            %if "&cov." = "SP(POW)(T)" %then %do;
                random intercept / subject=eyen(&animal.);
            %end;

Note that the macro variable &grp_no. may include sex as in your code, but that there is an opportunity to include interactions that may not be apparent in your model.

SteveDenham

Re: random statement

Ksharp — Wed, 07 Sep 2022 12:20:18 GMT

Kudos!
Thumbs Up!