When I ran a mixed model, I got a note of
NOTE: Convergence criteria met but final hessian is not positive definite.
I added "parms /old" and still got he same note. Is this because I used "type=cs" in the second random statement?
How to take care of this note message, or having this note does mean my model is not valid? Thanks
proc mixed data=addm1 method=reml;
class TRTPN(ref='1') VISITNUM(ref='1') SUBJID;
model CHG=TRTPN VISITNUM TRTPN*VISITNUM/solution;
random intercept/subject= SUBJID type=un;
random intercept/subject=pknum(SUBJID) type=cs;
lsmeans TRTPN/pdiff adjust=SCHEFFE;
parms /ols;
run;
There are several possible reasons, which are discussed in the excellent paper "Tips and Strategies for Mixed Modeling with
SAS/STAT Procedures" (Kiernan, Tao, and Gibbs, 2012). In my experience, a common reason is that the model does not fit the data, either because it is misspecified or because the sample is too small.
There are several possible reasons, which are discussed in the excellent paper "Tips and Strategies for Mixed Modeling with
SAS/STAT Procedures" (Kiernan, Tao, and Gibbs, 2012). In my experience, a common reason is that the model does not fit the data, either because it is misspecified or because the sample is too small.
Thank you Rick. I read this paper this morning, and the idea to include
parms /ols;
is from this paper. I will read it more carefully
Probably due to the small sample size, and we have 5 arms.
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