[R] correlation

Peter Dalgaard
Fri, 08 Feb 2008 16:37:42 -0000

darteta001 at ikasle.ehu.es wrote:
> Dear list
>
> I would like to compare two measurements of disease severity (M1 and
> M2), one of the is continuous (M1 ranging from 1 to 10) and the other
> is ordinal (M2 takes Low, Medium, high and very high). Do you think is
> ok to use cor() function to test whether the two agree, i.e correlate?
> I am afraid that if I set M2 to 1,2,3 and 4, the function cor() will
> take them as continuous and therefore lose intrepretation.
>
> Thanks for your commments
>
It's probably not massively wrong, given that the interpretation of
correlation coefficients is usually not very clear anyway (excepting
maybe nearly-perfect correlated cases). However, a Spearman correlation
does have the rather nice feature of being independent of the values you
assign to M2, and of any monotone transformation of M1 too.

--
   O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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