Greg Snow a écrit :
> One approach is to create your own contrasts matrix:
>
>   
>> mycmat <- diag(8)
>> mycmat[ row(mycmat) == col(mycmat) + 1 ] <- -1
>> mycmati <- solve(mycmat)
>> contrasts(agefactor) <- mycmati[,-1]
>>     
>
> Now when you use agefactor, the intercept will be the first age group and the slopes will be the differences between the pairs of groups (make sure that the order of the levels of agefactor is correct).
>
> The difference between this method and the contr.sdif function in MASS is how the intercept will end up being interpreted (and the dimnames).
>
> Hope this helps,
>
>   

Actually, exactly what I needed including the reference to contr.sdif in 
MASS I did not spot before (although I am a faithful reader of the 
yellow book... but so many things still escape to me). Again thanks a lot.

Patrick

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