Hi, I am having a hard time understanding the ANCOVA model and how to implement it in SAS. I am supposed to do an ANCOVA to obtain adjusted means and adjusted means differences for Y = change from baseline using treatment as a factor variable (3 categories), country as a categorical covariate (5 categories) and baseline as a continuous variable (hence the need for ANCOVA), the last two being adjustment variables. I have to use proc mixed and not proc glm or proc reg (Although I would love to know how it's done using these models as well). My SAS code looks something like: proc mixed data = mydata; ods output lsmeans = ls_means; class trt country; model change = trt country baseline; lsmeans trt / pdiff cov ; run; Is this correct? I also learned that an ANCOVA model requires the centering of the continuous variable. Is this something that has to be done manually or is it automatically done through SAS? I read somewhere that SPSS and R do it automatically. I know that the ANCOVA model looks something like this: How do I write my model in this form(knowing that I have an extra categorical adjustment variable)? Or can I just write it as a regression model: Change = Bo + B1 treat1 + B2 treat2 + B3 country1 + B4 country2 + B5 country3 + B6 country4 + B7 baseline + error (treat 3 = ref and country5 = ref)? One last thing : I am supposed to do a sequential comparison. Treat 1 vs Treat 3 Treat 2 vs Treat 3 Treat 1 vs Treat 2 How do I do that in SAS? Do I use the estimate options? I'm not too sure how that works either. Any and all help would be appreciated!
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