Joao,

There's a very thorough explanation at
http://www.ats.ucla.edu/stat/r/library/contrast_coding.htm

Jean


Joao Azevedo <joao.c.azevedo@gmail.com> wrote on 07/27/2012 06:32:31 AM:
> 
> Hi!
> 
> I'm failing to understand the value of the intercept value in a
> multiple linear regression with categorical values. Taking the
> "warpbreaks" data set as an example, when I do:
> 
> > lm(breaks ~ wool, data=warpbreaks)
> 
> Call:
> lm(formula = breaks ~ wool, data = warpbreaks)
> 
> Coefficients:
> (Intercept)        woolB
>      31.037       -5.778
> 
> I'm able to understand that the value of intercept is the mean value
> of breaks when wool equals "A", and that adding up the "woolB"
> coefficient to the intercept value I get the mean value of breaks when
> wool equals "B". However, if I also consider the tension variable in
> the model, I'm unable to figure out the meaning of the intercept
> value:
> 
> > lm(breaks ~ wool + tension, data=warpbreaks)
> 
> Call:
> lm(formula = breaks ~ wool + tension, data = warpbreaks)
> 
> Coefficients:
> (Intercept)        woolB     tensionM     tensionH
>      39.278       -5.778      -10.000      -14.722
> 
> I thought it would be the mean value of breaks when either wool equals
> "A" or tension equals "L", but that isn't true for this dataset.
> 
> Any clues on interpreting the value of intercept?
> 
> Thanks!
> 
> --
> Joao.

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