[R] Qualitative factors

[ZABALZA-MEZGHANI Isabelle]

Hi,
I have some doubt about how qualitative factors are coded in R. For
instance, I consider a response y, a quantitative factor x and a qualitative
factor m at 3 levels, generated as follow :

y_c(6,4,2.3,5,3.5,4,1.,8.5,4.3,5.6,2.3,4.1,2.5,8.4,7.4)
x_c(3,1,3,1,2,1,4,5,1,3,4,2,5,4,3)
m_gl(3,5)

lm(y~x+m)

Coefficients:
(Intercept)            x           m2           m3  
    3.96364      0.09818      0.44145      0.62291  

In literature, 2 usual implicit coding process are suggested : m1=0 or
m1=-m2-m3. Does R use one of these process ? (I've already read the R
documentation on this topic, and it is still not clear in my mind).
Furthermore, how can I make prediction using this model, I mean how should I
specify the new data (especially for m).

Thanks in advance for your help

Regards,

Isabelle Zabalza-Mezghani
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