Programming the statistical procedures from SAS

How to add a block variable in ANCOVA model

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Occasional Contributor
Posts: 6

How to add a block variable in ANCOVA model

[ Edited ]

Hi,

 

I am analyzing data from a randomized block design study. We have two independent variables (Treatment and Site), two covariate and one blocking factor (categorical). As I use a block design, I wonder what is a proper way to set up an ANCOVA model. I use SAS EG to do the analyses but I've told that  mayneed to modified the code myself to include a blocking factor. Do I just add one more factor in the classification variable option in Linear Model? How can I make sure that SAS would treat the block variable separately from the two independent variables I have? Below is my code from SAS EG.

 

PROC GLM DATA=work1;
CLASS Treatment Site Block;
MODEL Tb_BVTV= Treatment Site Treatment*Site Block Covariate(1) Covariate (2)
/
SS3
SOLUTION
SINGULAR=1E-07
;
LSMEANS Treatment Site Treatment*Site / PDIFF=ALL ADJUST=TUKEY ;
RUN;

 

 

Any advice would be appreciated.

Thank you!

Respected Advisor
Posts: 2,655

Re: How to add a block variable in ANCOVA model

You may have received an answer already, but this is a classic case where a mixed model approach is superior to PROC GLM.  See Littell et al.'s SAS for Mixed Models, 2nd ed. for examples.  In your case, I would suggest

 

PROC MIXED DATA=work1;
CLASS Treatment Site Block;
MODEL Tb_BVTV= Treatment Site Treatment*Site  Covariate(1) Covariate (2)
/

SOLUTION;
RANDOM Block;
LSMEANS Treatment Site Treatment*Site / DIFF=ALL ADJUST=TUKEY ;
RUN;

 

Steve Denham

 

 

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