Joao, There's a very thorough explanation at http://www.ats.ucla.edu/stat/r/library/contrast_coding.htm
Jean Joao Azevedo <joao.c.azev...@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. [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.