[R] predicting values from multiple regression

[Anna Lee]

Dear Ista!

Thank you for replying. The point you made is exactly what's the
problem: I want to predict the values at different points in space.
calP stands for the water content at each sampling point (n=90) but I
don't quite understand what R does. calP is my vector of measured data
and I thought with the predict function the programm would calculate a
value from the model function for every value of calP... ?

2011/3/20 Ista Zahn <izahn at psych.rochester.edu>:
> Hi Anna,
>
> On Sun, Mar 20, 2011 at 2:54 PM, Anna Lee <ana-lee at web.de> wrote:
>> Hey List,
>>
>> I did a multiple regression and my final model looks as follows:
>>
>> model9<-lm(calP ~ nsP + I(st^2) + distPr + I(distPr^2))
>>
>> Now I tried to predict the values for calP from this model using the
>> following function:
>>
>> xv<-seq(0,89,by=1)
>> yv<-predict(model9,list(distPr=xv,st=xv,nsP=xv))
>
> The second argument to predict.lm is newdata, which should be a
> data.frame. see ?predict.lm.
>
> Beyond that though, I'm not sure what you are trying to accomplish.
> The way you've set this up you would get predicted values for cases
> like
>
> distPr     st     nsp
> 0            0      0
> 1            1      1
> 2            2      2
> .             .       .
> 89          89     89
>
>
> Is that really what you want?
>
> Best,
> Ista
>>
>> The predicted values are however strange. Now I do not know weather
>> just the model does not fit the data (actually all coefficiets are
>> significant and the plot(model) shows a good shape) or wether I did
>> something wrong with my prediction command. Does anyone have an
>> idea???
>>
>> --
>>
>>
>> Thanks a lot, Anna
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>
>
> --
> Ista Zahn
> Graduate student
> University of Rochester
> Department of Clinical and Social Psychology
> http://yourpsyche.org
>



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