Hi all,

I am doing nls regression of the Rogers type III equation. However I can't 
find good starting values and keep getting the error messages: "Missing 
value or an infinity produced when evaluating the model" OR "singular 
gradient matrix at initial parameter estimates"
Is there a way to automatically check different starting values and give a 
list of values for each parameter? In my model only Th3 is a parameter 
with a good estimate (2.5). for v,w and x I would like to check a broad 
range of values. Is it possible to do this in R?

Under my script for plots and nls regression and a copy of the values of 
my data file.

plot(FR~N0,main="Rogers III")
RogersIII <- 
nls(FR~N0*(1-exp(((v+w*FR)/(1+x*FR))*(Th3*FR-24))),start=list(v=0.002,w=0.00075,x=-0.1,Th3=2.5),control=list(maxiter=1000000,minFactor=0.0000000000000000000000000000000000001)) 
# 24 is tijd in uren
hatac <- predict(RogersIII)
lines(spline(N0,hatac))
summary(RogersIII)

N0      FR
5       4
3       2
5       3
5       4
6       4
5       4
4       2
5       4
6       5
5       3
10      5
14      7
12      6
17      9
10      4
16      8
15      8
14      5
15      6
15      5
19      10
20      11
22      10
21      12
25      12
26      11
25      10
25      12
25      11
24      9
35      12
33      12
35      13
36      12
31      12
37      12
39      13
36      14
35      12
35      10
46      11
44      10
41      12
43      14
45      12
48      11
45      13
45      9
49      12
45      11
56      12
59      15
52      11
54      10
51      12
55      12
61      13
55      12
54      10
55      12

Kind regards,
Met vriendelijke groeten,
Joachim

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______________________________________

Joachim Audenaert
Adviesdienst Gewasbescherming
Proefcentrum voor Sierteelt
Schaessestraat 18
B-9070 Destelbergen
Belgium
Tel. +32 9 353 94 71
Fax +32 9 353 94 95
E-mail: joachim.audenaert@pcsierteelt.be
www.pcsierteelt.be
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