# [R] linear correlation?

Andrew Perrin andrew_perrin at unc.edu
Thu Mar 7 16:12:23 CET 2002

```On Thu, 7 Mar 2002, [iso-8859-1] dechao wang wrote:

> Thanks Andrew,
>
> Consider the following example:
> > x1<-c(1,  2,  3,   100, 200, 300)
> > x2<-c(1.1,2.8,3.3, 108, 209, 303)
> > x3<-c(2.8,3.8,5.3, 108, 209, 303)
> > cor(x1,x2)
> [1] 0.999655
> > cor(x1,x3)
> [1] 0.9997286
>
> You can see that as x2 changed to x3 with only first
> three numbers changing, the coefficients (x1, x2) and
> (x1,x3) changed little. I thought this may be because
> the last three numbers were in different units.

It's not because they're different units -- it's because they're different
measures altogether! Can you state, in words (e.g., not in mathematical
terms) what you think a correlation would *mean* between these two
vectors?  R is happily telling you, as any statistical package would, what
the correlation is between two vectors of numbers. But that correlation
doesn't necessarily mean anything at all; its meaning is based on what the
vectors measure.

----------------------------------------------------------------------
Andrew J Perrin - andrew_perrin at unc.edu - http://www.unc.edu/~aperrin
Assistant Professor of Sociology, U of North Carolina, Chapel Hill
269 Hamilton Hall, CB#3210, Chapel Hill, NC 27599-3210 USA

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```