[R] Comparison of age categories using contrasts

[Mark Difford]

Hi Patrick,

>> The default in glm is cont.treatment (for unordered factors) and that 
>> leads to compare each level to the first one. I would rather prefer to 
>> compare the 2nd to the 1st, the 3rd to the 2nd, the 4th to the 3rd,
>> etc...

The functions ?C and ?contrasts allow you to set up your own contrast
matrix. A function to carry out the type of comparisons you are interested
in has been written by Venables & Ripley. See contr.sdif in package MASS.

Regards, Mark.


Patrick Giraudoux wrote:
> 
> Dear listers,
> 
> I would like to compare the levels of a factor with 8 age categories 
> (0,10] (10,20] (20,30] (30,40] (40,50] (50,60] (60,70] (70,90] (however, 
> the factor has not been ordered yet). The default in glm is 
> cont.treatment (for unordered factors) and that leads to compare each 
> level to the first one. I would rather prefer to compare the 2nd to the 
> 1st, the 3rd to the 2nd, the 4th to the 3rd, etc... My understanding is 
> that cont.poly may make the trick, eg specified like this:
> 
> mod3<-glm(AE~agecat, family=binomial,data=qinghai2, 
> contrasts=list(agecat="contr.poly"))
> 
> but I am not sure to be right.
> 
> Would be grateful if a true statistician can confirm or fire me... and 
> before definitive fire tell me how to manage with this...
> 
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> 
> 

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