How (and why) do you use contrasts?
Under what cases do you create contrasts in your analysis? How is it done and what is it used for? I checked ?contrasts and ?C - both lead to "Chapter 2 of Statistical Models in S", which is not readily available to me. Contrasts are needed when you fit linear models with factors (i.e. categorical variables) as explanatory variables. The contrast specifies how the levels of the factors will be coded into a family of numeric dummy variables for fitting the model. Here are some good notes for the different varieties of contrasts used: http://www.unc.edu/courses/2006spring/ecol/145/001/docs