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Covatiance parameter estimate in Glimmix

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Covatiance parameter estimate in Glimmix

Hello,

I've been caught up in an issue in Proc Glimmix. For the same data, the same model and the same procedure, but I get different covatiance parameter estimate tables.

The treatment has 3 levels.

The model is :

proc  glimmix data=analysis initglm method=laplace;

class trial rep treatment;

model event/group_total=treatment;

nloptions tech=newrap maxit=400;

random trial rep(trial);

run;

Covariance parameter estimates
Cov Parm          Estimate                    Standard error

trial                 0                                    .                       
rep(trial)0.3120       .

Type III  Test of Fixed effect 

effect                         Num DF        Den DF         F value

treatment                 2               24               53.62      <.0001

However,if I sort the data by treatment first and re-run the glimmix procedure, I have different above tables:

Covariance parameter estimates
Cov Parm          Estimate                    Standard error

trial                      0                             0.2204    
rep(trial)0.31090.4130

Type III  Test of Fixed effect  

effect                         Num DF        Den DF         F value

treatment                   1                   24           162.42       <.0001

The df for numerator is 1. But the treatment has 3 levels, df should be 2.

Can anybody explain what causes these different results?  Is it because the model is over-specified? Thanks a lot!

Respected Advisor
Posts: 2,655

Re: Covatiance parameter estimate in Glimmix

Get a ticket opened with Tech Support right away.  Without access to your data, I can't really say what is going on, but that dropping of the treatment df makes me wonder if something has gone awry with the sorting, so check the log for that.  Also check the stuff that GLIMMIX reports early on, such as number of observations, number of columns in the X and Z matrices, etc.

I admit, something really odd is happening.

Steve Denham

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