parameterization of factor in R

Hi all,

I encountered this problem with parameterization in R:

I have two factors in a regression. how about if I want to
set constraint so that for each factor, the sum of their
coefficients equals to zero(instead of choosing a reference
category)? for example, I have factor(variable) A(with three
categories) and factor(variable) B(with 4 categories), and I want
to parameterize so that the sum of the three coefficients of
A = 0, and sum of the four coefficients of B = 0? I
mean how to do it in R?

Oana

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