[R] nlme leading minor error

[Petr Pikal]

Dear all

I am struggling with nlme and error message. Even going through 
Pinheiro, Bates nlme book did not gave me a clue how to avoid 
this.

fit <- nlme(ce ~ fi1 / ((1+exp(fi2-fi3*tepl))^(1/fi4)), data = 
temp1na.gr,
start = c(fi1=30, fi2=-100, fi3=-.05, fi4=40), 
fixed = fi1+fi2+fi3+fi4~1, 
random = pdDiag(fi2+fi4~1),
groups = ~spol.f)

gives

Error in chol((value + t(value))/2) : the leading minor of order 1 is 
not positive definite

Is this error due to lack of experimental points?
Here you have one typical part of my data. It is for level spol.f = 
"3/11".

tepl	ce
800	28.87
800	29.35
825	29
850	28.73
875	26.83
900	24.07

I have 1-5 points for each level (2 levels with 5 points, 1 level with 
4 points, several levels with 2 and 3 points and few with only one 
point.

Fitting this model to each level separately led to several sets of 
coeficients fi1-fi4 and the separate fits were quite OK.

Please give me a hint what can be the cause for this error message 
and how I shall organize my data to avoid this. (Lack of 
experimental points is also an answer as I can do some subsequent 
measurement.

R 2.1.0, W 2000, nlme package

Best regards



Petr Pikal
petr.pikal at precheza.cz