[R] correlation

[Peter Dalgaard]

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             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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