Proc Mixed, random intercept, intercept has zero degrees of freedom

Reply
Occasional Contributor
Posts: 6

Proc Mixed, random intercept, intercept has zero degrees of freedom

I'm looking at longitudinal data with proc mixed and random intercepts.  Each subject has 3 measurements (baseline, short term, long term).  There are two independent variables (time and surgery type), along with an interaction term (see code below).

When I run the model with random intercepts, the Solution for Fixed Effects shows an estimate, standard error and t-value for the intercept, but no degrees of freedom and (obviously) no p-value.

When I run the same model without random intercepts, then the SFE reports the appropriate degrees of freedom and p-value.

There are 21 subjects in the dataset, each with 3 measurements.  Is this an overparameterization issue?

Code:

proc mixed data=work.mydata order=data ;


  class surgery time;

  model outcome_score =   surgery|time /solution ;

  repeated /subject=mrn type=un r rcorr;

  random int;

run;

With random int, estimate for intercept = 57.2, SE = 7.1, DF = 0, t Value = 7.99, p-value = .

Without random int, estimate for intercept = 57.2, SE = 4.9, DF = 19, t Value = 11.66, p < .0001

Again, I think this is an overparameterization issue, but I'm not sure.  Any input would be appreciated.

Andrew

Ask a Question
Discussion stats
  • 0 replies
  • 177 views
  • 0 likes
  • 1 in conversation