

Adjusted means controlling for covari... 

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Dear Bengt and Linda: I am running two models: one ANOVAlike model where I use dummycoded variables for three of the four categories used, and the other an ANCOVAlike model where I add a continuous covariate in addition to these dummycoded predictors. I am getting means for the various groups by adding the coefficient for a given group to the intercept term. This works well for the model without the covariate, but in the model with the covariate I am getting adjusted means that are outside the range of possible scores for the dependent variable. Am I doing something wrong, or is this what should be happening? Again, thank you very much. Seth Schwartz 


The covariate mean may be different in the different groups. The beta*mean term for this covariate needs to be included when you compute the estimated means. 


Dear Bengt and Linda: I am also trying to compute adjusted means for the DV in an ANCOVAlike model. Model: heightav on group1; heightav on group2; heightav on group3; heightav on test_age; I don't understand what you mean by including the beta*mean term for this covariate, can you give an exemple? Thanks! Nadine 


Using your example, I assume you have 4 groups and the 3 group variables that you mention are 0/1 dummy scored where only one of them has a value 1 while the others are 0, and when all 3 are zero this refers to the 4th group. I also assume that test_age is continuous. Then what I meant was that the mean of heightav for group j (j=1 or 2 or 3) is intercept+betaj+beta*test_age, where betaj is the slope for heightav ON groupj and beta is the slope for heightav ON test_age. The mean of heightav for the 4th group is intercept+beta*test_age. 

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