[R] lm and R-squared (newbie)

[PtitBleu]

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?

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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