Hi Dear longitudinal addicts,
I have a quick issue to submit u that'll take u hopefully only a few
minutes:

I ran an lme model and I receive the following error mex that says a no
positive varcov matrix was estimated..
Now at the moment I don't exactly know the code implemented by Pinheiro and
Bates so I'm not able to say what was the mechanism that could possibly
generate such a problem.

First thing I thinking from scratch is that te two bolded variables in the
model below (cd4, log(rna)) could actually be higly (negatively) correlated
so collinearity may occurr and wreak havocs there..

By removing cd4 the problem disappear indeed.

I would still ask u an opinion if that won't cost u much....



> RATIO1C2<-lme(ratio1~rt+rt*(rt>=25)+*cd4*+cd4:rt+
+ cd4:rt*(rt>=25)+resp2+resp2:rt+resp2:rt*(rt>=25)+
+ log(rna)+*log(rna)*:rt+log(rna):rt*(rt>=25)+
+ sex+age+rf+nadir+pharmac2+pharmac2:rt+
+ hbs+hcv+haartd+hivdur+switch,
+ subset=which(ratio1<10),data=frame4b,
+ correlation=corARMA(p=4),
+ random= ~rt|id, na.action=na.omit)
>
> intervals(RATIO1C2)$fixed
*
Errore in intervals.lme(RATIO1C2) :
  Cannot get confidence intervals on var-cov components: Non-positive
definite approximate variance-covariance
*
thank u indeed

Freddy



-- 
*Little u can do against ignorance,....it will always disarm u:
is the 2nd principle of thermodinamics made manifest, ...entropy in
expansion.**....But setting order is the real quest 4 truth, ......and the
mission of a (temporally) wise dude.
*

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