Dear List! How can I define contrasts (design matrix) that can all be included, i.e. which do not require a control category be dropped. My application (see below) does not suggest a sensible control category. I am thinking of constraining the (treatment) contrasts to sum up to zero and dropping the constant term in the regression. Is this a good idea? If yes, how to achieve this in R? I am estimating a GLM for bilateral country data. Each observation in on a pair of countries, e.g. GER_USA, GER_JAP, USA_JAP. I constructed the following contrasts: d_GER, d_USA, d_JAP, which take the value of 1 when the country is in the pair and 0 otherwise, i.e. ?Bilat?, ?d_GER?, ?d_USA?, ?d_JAP? GER_USA, 1, 1, 0 GER_JAP, 1, 0, 1 USA_JAP, 0, 1, 1 These contrasts highlight the effect of having a given country in the pair. Thank you for your help! Serguei Kaniovski