[R] Odp: An error in fitting a non linear regression

[Petr PIKAL]

Hi

r-help-bounces at r-project.org napsal dne 20.02.2009 11:21:26:

> 
> Hi
> 
> I have a data set with two variables "q" and "depth" as follows:
> 
> q<-c(tapply(weight[Soil=="Jy"], Depth[Soil=="Jy"], mean)). This commns
> returns 7 "q" values:
> 0.687900000 0.845550000 0.405416667 0.152766667 0.033100000 0.031400000
> 0.005183333
> 
> The "depth" values are produced using this command whish "depth" shows 
teh
> soil depth: 
> depth <--c(5,15,25,35,45,55,65). This return these values for "depth":
> [1]  -5 -15 -25 -35 -45 -55 -65
> 
> I have uploaded the plot file in below:
> 
> 
> 
> http://www.nabble.com/file/p22118160/plot.doc plot.doc 
> 
> Now I want to fit the following decaying exponential model to our data
> presented in the plot file such that depth is X and q is Y.
> nreg<-nls(q~a*exp(-b*depth), start=list(a=1, b=0.5)). This command 
returns
> the following error:
> Error in nls(q ~ a * exp(-b * depth), start = list(a = 1, b = 0.5)) : 
>   singular gradient

With b=0.05 it will converge, however the model seems to be wrong.

depth <--c(5,15,25,35,45,55,65)
y<-c(0.687900000, 0.845550000, 0.405416667, 0.152766667, 0.033100000, 
0.031400000,0.005183333)
plot(depth,y)
nreg<-nls(y~a*exp(-b*depth), start=list(a=1, b=0.05))
lines(depth, predict(nreg))

Regards
Petr


> 
> However, if I use positive values of depth i.e. 
> [1]  5 15 25 35 45 55 65
> 
> then I recieve the following error:
> Error in numericDeriv(form[[3]], names(ind), env) : 
>   Missing value or an infinity produced when evaluating the model
> 
> Notice that though in the plot depth is drawn on Y axis but in the
> regression model "depth" in independent. Plot shows the real world of 
what
> happne to the crop root growth.
> 
> Thank you.
> Saeed Ahmadi
> 
> -- 
> View this message in context: 
http://www.nabble.com/An-error-in-fitting-a-non-
> linear-regression-tp22118160p22118160.html
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> 
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