[R] lm and R-squared (newbie)

[David Winsemius]

On Dec 15, 2011, at 8:35 AM, PtitBleu wrote:

> Hello,
>
> I've two data.frames (data1 and data4), dec="." and sep=";".
> http://r.789695.n4.nabble.com/file/n4199964/data1.txt data1.txt
> http://r.789695.n4.nabble.com/file/n4199964/data4.txt data4.txt
>
> When I do
> plot(data1$nx,data1$ny, col="red")
> points(data4$nx,data4$ny, col="blue")
> ,  results seem very similar (at least to me) but the R-squared of
> summary(lm(data1$ny ~ data1$nx))
> and
> summary(lm(data4$ny ~ data4$nx))
> are very different (0.48 against 0.89).
>
> Could someone explain me the reason?

Because you failed to do an adequate assessment of your data. Try this  
ploting exercsie and I think you will see the reason for the  
differences:

plot(data1$nx,data1$ny, col="red", xlim=range(c(data1$nx,data4$nx)),  
ylim=range(c(data1$ny,data4$ny)) )

-- 
David.

>
> To be complete, I am looking for an simple indicator telling me if  
> it is
> worthwhile to keep the values provided by lm. I thought that R- 
> squared could
> do the job. For me, if R-squared is far from 1, the data are not  
> good enough
> to perform a linear fit.
> It seems that I'm wrong.
>
> Thanks for your explainations.
> Ptit Bleu.
>
>
>
>
>
> --
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>
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David Winsemius, MD
West Hartford, CT