[R] Comparison of age categories using contrasts
mark_difford at yahoo.co.uk
Mon Feb 16 08:02:53 CET 2009
>> 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,
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.
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,
> 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...
> R-help at r-project.org mailing list
> PLEASE do read the posting guide
> and provide commented, minimal, self-contained, reproducible code.
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