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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