Example 55.4 Reading Mixed Model Results from PARMS= and COVB= Data Sets

This example creates data sets containing parameter estimates and covariance matrices computed by a mixed model analysis for a set of imputed data sets. These estimates are then combined to generate valid statistical inferences about the parameters.

The following PROC MIXED statements generate the fixed-effect parameter estimates and covariance matrix for each imputed data set:

proc mixed data=outmi;
   model Oxygen= RunTime RunPulse RunTime*RunPulse/solution covb;
   by _Imputation_;
   ods output SolutionF=mixparms CovB=mixcovb;
run;

The following statements display (in Output 55.4.1) output parameter estimates from PROC MIXED for the first two imputed data sets:

proc print data=mixparms (obs=8);
   var _Imputation_ Effect Estimate StdErr;
   title 'MIXED Model Coefficients (First Two Imputations)';
run;

The following statements display (in Output 55.4.2) the output covariance matrices associated with the parameter estimates from PROC MIXED for the first two imputed data sets:

proc print data=mixcovb (obs=8);
  var _Imputation_ Row Effect Col1 Col2 Col3 Col4;
  title 'Covariance Matrices (First Two Imputations)';
run;

Note that the variables Col1, Col2, Col3, and Col4 are used to identify the effects Intercept, RunTime, RunPulse, and RunTime*RunPulse, respectively, through the variable Row.

For univariate inference, only parameter estimates and their associated standard errors are needed. The following statements use the MIANALYZE procedure with the input PARMS= data set to produce univariate results:

proc mianalyze parms=mixparms edf=28;
   modeleffects Intercept RunTime RunPulse RunTime*RunPulse;
run;

The "Variance Information" table in Output 55.4.3 displays the between-imputation, within-imputation, and total variances for combining complete-data inferences.

The "Parameter Estimates" table in Output 55.4.4 displays the estimated mean and standard error of the regression coefficients.

Since each covariance matrix contains variables Row, Col1, Col2, Col3, and Col4 for parameters, the EFFECTVAR=ROWCOL option is needed when you specify the COVB= option. The following statements illustrate the use of the MIANALYZE procedure with input PARMS= and COVB(EFFECTVAR=ROWCOL)= data sets:

proc mianalyze parms=mixparms edf=28
               covb(effectvar=rowcol)=mixcovb;
   modeleffects Intercept RunTime RunPulse RunTime*RunPulse;
run;