Message-ID: <742479270809020529h7bfa410brbf84578354939e6a@mail.gmail.com>
Date: 2008-09-02T12:29:31Z
From: R.S. Cotter
Subject: Error: "Cannot get confidence intervals...", with lme, what does it means?
Hello,
In some occasions I get this error message: "Cannot get confidence
intervals on var-cov components: Non-positive definite approximate
variance-covariance".
I have tried to figure out this by using help function, but didn't
find answer to the question. I address this question with describing
the model and the primary task that I want to solve. Sorry if the
question is clumsy formulated, I 'm not that experienced with R and
statistics.
My model is:
Response= Weight(continous)
Explanatory variables= Time (continous) and Diet (kategorical, two groups; B&C)
The primary question of interest is wheter the growth rates
(Weight/Time) differ among the two diets.
lmefit1<-lme(Weight ~ Diet*Time,random=~1|Place,data=Total)
Summary output is ok, so far so good. But I also wanted to get the
slope and confidence intervals for the growth rates for both diets
(B&C), so I ran intervals(). And I got the intercept, slope and
confidence intervals for diet B, see below.
But I also wanted the same for the diet C, to do this I renamed diet C
to A in the data sheet to force C to be the dummy variable. Is this
the right way to do it?
When running the intervals () once again, I got this message: "Cannot
get confidence intervals on var-cov components: Non-positive definite
approximate variance-covariance". What could be wrong..? Is there
other ways to get the slope and confidence intervals from a lme model?
> intervals(lmefit1)
Approximate 95% confidence intervals
Fixed effects:
lower est. upper
(Intercept) 66.040673 108.122242 150.203810
DietC -175.080336 -109.638518 -44.196700
Time 4.177387 5.434087 6.690788
DietC:Time 7.938101 11.180806 14.423511
attr(,"label")
[1] "Fixed effects:"
Random Effects:
Level: Place
lower est. upper
sd((Intercept)) 0.1478599 13.50651 1233.775
Within-group standard error:
lower est. upper
159.9128 174.8928 191.2761
Best regards Cotter