[R] Interactions in a nls model

[Anthony Lawrence Nguy-Robertson]

Gabor,

Looking at your 2nd response, your suggestion is similar to that of 
Derek Ogle. It was my misinterpretation from your first response that 
elicited my original rebuttal. I apologize.

Thanks,
Tony

On 2/9/2011 2:47 PM, Gabor Grothendieck wrote:
> On Wed, Feb 9, 2011 at 1:02 PM, Anthony Lawrence Nguy-Robertson
> <anthony.robertson at huskers.unl.edu>  wrote:
>    
>> Thank you R-forum for you generous help.
>>
>> Gabor Grothendieck, I am not sure if anova in the form that you suggested is
>> the most appropriate (This is probably more of a statistics related, rather
>> than R related at this point). The way I understand anova is that you are
>> testing the variance between the models. I know that the variance in the
>> 'corn' models is greater than 'soybean' due to a biological reason. I don't
>> think this approach is correct in my case since the anova analysis is
>> comparing variance between different data sets.
>>      
> Yes, you do an F test between the models using anova.nls except the
> models I specified in my response are not the models that you used and
> I think you are misinterpreting my answer even though I did explicitly
> point out that the models I was referring to were not the models in
> your post.  The models you used can't be compared this way since they
> each refer to a different data set.  I corrected the approach in your
> question by using nested models which both refer to the same combined
> data set.
>
> "Just to be clear the two models would each include both groups -- one
> model would assume the parameters are the same for the two groups so
> it would have 3 parameters and the other model would allow them to be
> different (up to 6 parameters depending on how many parameters you
> wish to allow to be different between the two groups) -- these are not
> the models shown above."
>
>    

-- 
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
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Office: 402-472-2565
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Email: anguy-robertson at unlnotes.unl.edu