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...t separate regression analysis for each level of var1 and expt with the command: by(data3, data3$var1:data3$expt, function(x) lm(diameter ~ day, data = x)) How can I get residual plots for each combination var1*expt generated above? Thanks! Eduardo -- Eduardo S. G. Mizubuti Departamento de Fitopatologia Universidade Federal de Vi?osa 36570-000 Vi?osa, MG Brasil tel.: +55 31 3899 1090 fax: +55 31 3899 2240 e-mail: mizubuti at ufv.br