# Re: [R] How to Get Categorical Correlation Coefficient

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Thu 12 Oct 2006 - 08:25:33 GMT

> Howdy Gurus !
>
> I have a different correlation result from the same data. The
> "corridor1" string variable is expressed
> as a number like the "corridor2" number variable.
> --------------------------------------------------------------------------
> > levels(corridor1)
> [1] "A" "B" "C" "D" "E" "F"
> > levels(as.factor(corridor2))
> [1] "0" "1" "2" "3" "4"
> >
> ------------------------------------------------------------------------------------------
> I have the correlation results followings using cor() function.
> ------------------------------------------------------------------------------------------
> > cor(jh1_1, as.factor(corridor1))
> [1] 0.01528538
> > cor(jh1_1, as.factor(corridor2))
> [1] -0.4972571
> ------------------------------------------------------------------------------------------
> I donot know why the above correlation coefficients used the same data
> are different.
> They are 0.015 from as.factor(corridor1), -0.497 from as,factor(corridor2).
> The string variable "corridor1" is the same catergory data with the
> variable corridor2.
> The difference is that "A" is replaced with "0", "B" with "1", "C"
> with "2", .....
>
> Could you tell me why they are different, and which correlation
> coefficient is correct?

One thing that strikes me is that corridor1 has 6 levels and corridor2 has 5...

In general correlations are not expected to work on factors so I'd be explicit about taking as.numeric(). A glance at table(corridor1,corridor2) should be informative too, as would a summary(as.numeric(as.factor(corridor1))-as.numeric(as.factor(corridor1)))

```--
O__  ---- Peter Dalgaard             Ă˜ster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)                  FAX: (+45) 35327907

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