Hi All,
I'm trying to run nls on the data from the study by Marske (Biochemical
Oxygen Demand Interpretation Using Sum of Squares Surface. M.S. thesis,
University of Wisconsin, Madison, 1967) and was reported in Bates and Watts
(1988).
Data is as follows, (stored as mydata)
time bod
1 1 0.47
2 2 0.74
3 3 1.17
4 4 1.42
5 5 1.60
6 7 1.84
7 9 2.19
8 11 2.17
I then run the following;
#Plot initial curve
plot(mydata$time, mydata$bod,xlab="Time (in days)",ylab="biochemical oxygen
demand (mg/l) ")
model <- nls(bod ~ beta1/(1 - exp(beta2*time)), data =
mydata, start=list(beta1 = 3, beta2 = -0.1),trace=T)
The start values are recommended, (I have used these values in SPSS without
any problems, SPSS returns values of Beta1 = 2.4979 and Beta2 = -2.02 456)
but return the error message,
Error in nls(bod ~ beta1/(1 - exp(beta2 * time)), data = mydata, start =
list(beta1 = 3, : singular gradient
Can anyone offer any advice?
Thanks in advance
Mike
--
Michael Pearmain
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