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Hi folks, I'd just like to ask a quick query. I hope that someone can shed some light. For an ANCOVA with one continuous dependent variable, one continuous covariate and a categorical q level independent variable (where each level represents a 'group') we are able to generate the group means which have been adjusted for the covariate and test the difference between group means adjusted for the covariate. ANCOVA is basically a regression model. Say if we have a 3 level 'group' variable (B) with level 3 as the reference group and the covariate is called C we will have: Y = constant + coeffc * C + coeffb1*B_level1 + coeffb2*B_level2 ...and to calculate the adjusted mean for each group, we enter the average value of C. As I understand, we are assuming (above) that the overall relationship between Y and C is the same for all groups - that is why there is only one coefficient for C. Thus one of the major assumptions of ANCOVA is that there is 'homogeneity of regression slopes' and this is tested via the B*C interaction. If we considered this as a multiple regression (with a q level categorical predictor B coded as q-1 dummy binary variables and a continuous predictor C) rather than an ANCOVA, we would not do a test of homogeneity of regression slopes.....why is this so? Many thanks for your views. Kind Regards, Kim Dr Kim Pearce PhD, CStat, Fellow HEA Senior Statistician Haematological Sciences Room MG261 Institute of Cellular Medicine William Leech Building Medical School Newcastle University Framlington Place Newcastle upon Tyne NE2 4HH Tel: (0044) (0)191 208 8142 You may leave the list at any time by sending the command SIGNOFF allstat to [log in to unmask], leaving the subject line blank.