[R] non linear regression with nls

[lauramorgana at bluewin.ch]

Hello, thanks for the advice of nlsList!
I tried to look at the help page of nlsList, but I didnt understand how to use the subset argument of the function 
and it's not clear to 
me if this only allows you to choose one subset or if it run the regression for every given subset, in this case how 
can someone specify the groups/subset?
Thanks a lot!
Laura


----Messaggio originale----
Da: kfrost at wisc.edu
Data: 03.02.2009 15.36
A: <lauramorgana at bluewin.ch>
Oggetto: Re: [R] non linear regression with nls

Hi, Laura-

You might have a look at ?nlsList().

Ken


----- Original Message -----
From: "lauramorgana at bluewin.ch" <lauramorgana at bluewin.ch>
Date: Tuesday, February 3, 2009 4:38 am
Subject: [R] non linear regression with nls
To: r-help at r-project.org


> Hello,
> I'm a beginner with R and it's the first time I'm using the R-help 
> list... I hope I'm in the right place, if not: 
> Sorry!!
> 
> I need to do non linear regressions on a data set which columns are:
> "river.name"    "Portata"  "PTG.P"   "PO4.P"   "NT.N"    "NH4.N"   
> "NO3.N"   "BOD5"    "SiO2"   
> I need to predict every variable (PTG, PO4, NT, ..., which are 
> concentration of substances in water) starting from 
> the "Portata" variable (which is the water flow)
> The functions that I'm using are:
> > fz1<-function(Portata, a, b){a+b/Portata}
> > fz2<-function(Portata, a, b){a*exp(b*Portata)}
> > fz3<-function(Portata, a, b, d, e){a+b/Portata+d*(Portata^e)}
> > fz4<-function(Portata, b, d){b*Portata^d}
> > fz5<-function(Portata, a, b, d){a+b*(Portata^d)}
> I've made a list of the functions with list(fz1, fz2, fz3, fz4, fz5)
> 
> and the starting , lower and upper parameters for each function are:
> fz1: start=list(a=10, b=10), lower=list(a=0, b=0), upper=list(a=1000, 
> b=1000) 
> fz2: start=list(a=10, b=1), lower=list(a=0, b=0), upper=list(a=1000, b=1000)
> fz3: start=list(a=10, b=10, d=10, e=10), lower=list(a=0, b=0, d=0, 
> e=-50), upper=list(a=1000, b=1000, d=1000, e=50) 
> fz4: start=list (a=10, b=1), lower=list(a=0, b=-50), 
> upper=list(a=1000, b=50)
> fz5: start=list(a=10, b=10, d=1), lower=list(a=0, b=0, d=-50), 
> upper=list(a=1000, b=1000, d=50)
> 
> so far i manage to do non linear regression one at a time that is, 
> using one function for one river) using nls(), for 
> example:
> r.NT.lav<-nls(NT.N~ fz1(Portata, a,b), 
> data=subset(dati,river.name=="Laveggio"), start=list(a=10, b=10), nls.control
> (maxiter=200), algorithm='port', trace=TRUE, na.action=na.omit, 
> lower=list(a=0, b=0), upper=list(a=1000, b=1000))
> and then I get the results with summary() and the graph using plotfit()
> 
> but this will get extremly long since I have 12 rivers to analize for 
> every variable and then compare the results, so 
> I'd like to use a loop (cycle for??) but I can't figure out how it 
> works. I've tried to look on the help page on ?
> Control (control flow) but I didn't understand it... 
> Can somebody help me (give me a hint or an example of a loop) to 
> automize the regression and save the results
> Please consider that my knowledge of computer programming is 
> practically non-existent!!
> Thanks a lot!
> Laura  F.
> 
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