Re: [R] Interactions in a nls model

Anthony Lawrence Nguy-Robertson Wed, 09 Feb 2011 11:08:11 -0800

Thank you R-forum for you generous help.

Gabor Grothendieck, I am not sure if anova in the form that yousuggested is the most appropriate (This is probably more of a statisticsrelated, rather than R related at this point). The way I understandanova is that you are testing the variance between the models. I knowthat the variance in the 'corn' models is greater than 'soybean' due toa biological reason. I don't think this approach is correct in my casesince the anova analysis is comparing variance between different data sets.

However, using the approach suggested by Derek Ogle seems to produceresults that make more sense since the anova analysis is between theentire data set and the influence of crop type on that model. For thecouple of models I tested thus far, I think this is the approach I needto take. I am not a statitician, so I hope my descriptions make sense.Again I appreciate everyone's help in this matter.


Thanks,
Tony

On 2/9/2011 7:53 AM, Derek Ogle wrote:

Anthony,

The link below shows how I did a similar analysis but for a different nls 
model.  Perhaps it will be useful to you ...

http://www.ncfaculty.net/dogle/fishR/gnrlex/VonBertalanffyCompare/VonBertalanffyCompare.pdf

Good luck.

-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-bounces@r-
project.org] On Behalf Of Gabor Grothendieck
Sent: Tuesday, February 08, 2011 10:43 PM
To: Anthony Lawrence Nguy-Robertson
Cc: r-help@r-project.org
Subject: Re: [R] Interactions in a nls model

On Tue, Feb 8, 2011 at 4:12 PM, Anthony Lawrence Nguy-Robertson
<anthony.robert...@huskers.unl.edu>  wrote:

I am interested in testing two similar nls models to determine if the

lines

are statistically different when fitted with two different data sets;

one

corn, another soybean. I know I can do this in linear models by

testing for

interactions. See Introductory Statistics with R by Dallgaard p212-

218 for

an example. I have two different data sets I am comparing to lai.

ci.re

should have very little difference between corn and soybean, while

ci.gr,

there should be a difference. If I use the simplistic form described

in

Dallgaard (see example in code below) it does not work correctly.

What do I

need to do to test for this interaction? Thank you!

My data is located here: ftp://snrftp.unl.edu/Outgoing/example/

Here is my example code:

###load data###
data<- read.csv("example.csv", header=TRUE)
eq<- function(x,a,b,c) {a+b*exp(x^c)}

##create non-linear models##
nls.ci.re<- nls(lai~eq(ci.re,a,b,c),data=data, start=c(a=1,b=1,c=1))
nls.ci.gr<- nls(lai~eq(ci.gr,a,b,c),data=data, start=c(a=1,b=1,c=1))

#create non-linear models for corn#
nls.ci.re.corn<-

nls(lai~eq(ci.re,a,b,c),data=subset(data,crop=="corn"),

start=c(a=1,b=1,c=1))
nls.ci.gr.corn<-

nls(lai~eq(ci.gr,a,b,c),data=subset(data,crop=="corn"),

start=c(a=1,b=1,c=1))

#create non-linear models for soybean#
nls.ci.re.soybean<-
nls(lai~eq(ci.re,a,b,c),data=subset(data,crop=="soybean"),
start=c(a=1,b=1,c=1))
nls.ci.gr.soybean<-
nls(lai~eq(ci.gr,a,b,c),data=subset(data,crop=="soybean"),
start=c(a=1,b=1,c=1))

#test interactions according to Introductory Statistics with R by

Dalgaard

p. 213#
corn.soybean.interactions.ci.re.a<-
    abs((summary(nls.ci.re.corn)$coeff[1,1]-
    summary(nls.ci.re.soybean)$coeff[1,1])/
    sqrt(summary(nls.ci.re.corn)$coeff[1,2]^2+
    summary(nls.ci.re.soybean)$coeff[1,2]^2))

corn.soybean.interactions.ci.gr.a<-
    abs((summary(nls.ci.gr.corn)$coeff[1,1]-
    summary(nls.ci.gr.soybean)$coeff[1,1])/
    sqrt(summary(nls.ci.gr.corn)$coeff[1,2]^2+
    summary(nls.ci.gr.soybean)$coeff[1,2]^2))

corn.soybean.interactions.ci.re.a.p.value<-

pt(corn.soybean.interactions.ci.re.a,df=summary(nls.ci.re.corn)$df[2],l
ower.tail=FALSE)

corn.soybean.interactions.ci.gr.a.p.value<-

pt(corn.soybean.interactions.ci.gr.a,df=summary(nls.ci.gr.corn)$df[2],l
ower.tail=FALSE)

There is an anova.nls method: anova(model1, model2)


--
Statistics&  Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com

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--
Anthony L. Nguy-Robertson
Doctoral Student
University of Nebraska-Lincoln
School of Natural Resources
Center for Advanced Land Management Information Technologies (CALMIT)
223 Hardin Hall
3310 Holdrege Street
Lincoln, NE  68583
Office: 402-472-2565
Fax: 402-472-2946
Email: anguy-robert...@unlnotes.unl.edu

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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

Re: [R] Interactions in a nls model

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